I believe it is important to maintain constant rms levels. But of course still use your peak measurement metering at the same time. 
 
If the main concern is to avoid clipping then one great way to do it is to maintain a constant rms level eg -20 dB FS. Keep all the rms levels on tracks, buses and the final mix the same eg -20 dB rms = 0 dB VU.  If you do this you are directly transferring a metering concept from analog to digital and it still works the same.  Peaks take care of themselves and you can get on with other things.  The rms to peak ratio varies all over the place but I say ignore it and let the built in headroom handle it.
 
If you start seeing clipping lights on then often it is a simple matter to reduce the rms level slightly.  Treat it with a limiter somewhere and bring the transient back under 20 dB. If you do see a clip light even with the rms metering approach it will usually be so short lived you won’t hear it either.  (Sometimes it can sound good or better)
 
Your peak metering comes in handy and in conjunction with rms metering you have got it all covered.  Keeping rms levels consistent throughout every part of the signal chain makes for very easy production. You have tracked everything perfectly so you always have the right amounts of available level for the mix stages. Your buss mixes are all sitting at the same rms level as well as your final mix.  Every mix will be the same volume too before any mastering begins. 
 
rms levels tie in perfectly with loudness measurements and dynamic range measurements. Peak levels are related to the rms parts they are attached to. They are important. They add the snap to the sound and give us the all important dynamics.  Peak metering is fast and it lets us see how high those transient peaks really are.
 
Pick an rms level and put everything you do rms wise right there at that level.  Link you room SPL level to that level as well.  Drop everything you do down and you will naturally move away from 0 dB FS.  Turn the level of your monitoring up and hear how perfect everything can sound.  All because you are keeping an eye now on both components of the signal.  rms and peak.  And now keeping the rms component at a ref level like we used to.  I see it as the rms component representing the body of the signal.  The peak reading the transient component of the signal.
 
Nothing has changed much even in our digital recording world.  All the source signals are the same, mics on drums, guitars, vocals percussion you name it.  Synths (hardware) are all analog outputs mostly.  There is so much analog that still exists well prior to the digital recording medium.  So it pays to keep an analog metering concept alive to meter that and why not continue to meter that way right through the digital medium and out the other end.  It works for me and sounds great.  Digital sounds great when you operate it this way too.

I should point out that if you record at the -12 to -18 db levels with no overages as suggested you really will not need to do anything with the gain control because you will not have to attenuate the recorded material, its already where you need it to be. You can then use your effects output controls  to maintain the proper levels throughout the PC and effects. I should also point out I am in no way an expert, this is just how I try to do it.

It all starts with tracking and the good thing about thinking ref level concepts is you start by tracking there. Most incoming signals will make a VU meter move well (and the ones that don't require more use of the peak metering) so set it right with tracking.  Get that VU just hitting 0 dB VU nicely and you know you are tracking the input signal exactly at the rms ref level. Many virtual instruments have wildly swinging output levels.  Metering the correct rms ref level leaving them always ensures bounces will be at the perfect level too.  And you will always have the perfect amount available for any mix.
 
Peak levels will always vary in relation to rms levels. Some peaks might only be 6 dB above the rms component, others might be 12 dB above the rms component.  Keeping the rms component consistent means no gain adjustments will ever be required prior to a mix either.
 
If you have a plug in effect make sure the rms level is correct just prior. VU meter plugins can be inserted anywhere. There is always some place you can set gain prior to the point of interest.  If you have a plug in chain ensure the rms level is right just prior to it.
 
The signals leaving a plug-in/chain should be at the same rms level as going in.  Levels leaving can always be tweaked by the output level control of the plugin or the last plugin in the chain.  If you work at a modest rms ref level such as -14 or -20 you will never clip any plugin either going in, internally or coming out.  You immediately get the best performance from them.  Some plugs benefit from higher than normal rms levels going in or during but it is always easy to reset that on output.
 

Is there a way to Set the track meters in Sonar to ( Pre ) before the fader.
like in a real analog mixing desk, where you have the gain knob and a red led to watch
the Gain input ( clipping led )

Wont help to set " gain "  if the indicator... that is the meters, are affected by the fader level

We need a Pre / post, fader level switch in the Mixer panel below the meter
with a color change in the meter to dark red when in pre mode !

or have i missed something here.....

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Edit found this in console view : options \ meters \ record meter options ......  but there's no post / Pre option

When you're recording, the wave displays in the track.  This is a direct representation of the stream of numbers being output by your interface.  If your interface "peaks out", ie, the input voltage goes over the top limit for the A/D and produces a stream of numbers at max value, this is highlighted in the waveform display.  The only cure is to turn down the input gain on your interface and record again.
The pre/post meter switches on an old analogue desk are so you can measure the level that's going to aux busses that are set to pre.  The signal level going to tape is post fader, the fader being used to set the record level.
 
 
 
Edit:  Actually, no.  That is a use,  but a meter set to pre is mostly so you get a constant view of what is happening on the track, even if the fader is pulled right down or the track is muted.   Regardless, it's nothing to do with recording levels or even gain staging.

There's so much conflicting information out on the web and You Tube, and I think alot of it has to do with the fact that Sonar is one of the few DAWs to actually have a gain control permanently on the mixing view.  Almost every other DAW out there requires a trim plugin in the FX chain.  There are a ton of videos out there showing people how to do gain staging with the faders, which as many of you have already pointed out is completely the opposite of the signal path.
 
I see a few folks with Cubase and Studio One use clip gain to set the gain control (I think PT recently introduced this as well), but to me that's much more tedious than just using a gain plugin or as Sonar has so smartly laid out a gain knob, because that's what you'd have on an actual mixing desk!
 
 

+1 to Sonar having a clearly visible gain knob on every track. I miss this so much when working in Logic.  Logic's meters also leave a lot to be desireD when it comes to gain staging.

Most meters on most DAW's are useless at showing you rms levels as well.  Sonar is one of the worst as it shows the rms level being 3 db down from where it actually is.  Also seeing some rms value way down low on a meter scale is also not helpful either.
 
When you are at your ref level (digitally that is) the VU meter needs to read 0 dB VU or close to full scale deflection.  Studio One has a mode where the buss meters can be put into this mode at anytime at the K system ref levels.
 
You don't need any gain trim controls at all if you track correctly using a VU meter.  All your tracks will simply be at the correct rms level before you go any further.
 
Clip gain in PT and Studio One is not really a gain trim control either.  If you track correctly you won’t need to adjust that but it is nice and handy for trimming sections here and there.  Also because the waveform height does change (and this is the right way BTW, the only way)  it is easy to match things by eye and get very close.
 
Studio One also allows you to meter the incoming signal just as it arrives at the sequencer before it even goes into a track.  You can open the inputs and insert any meter plug on there.  I thought I would not use it much but it is a very cool feature.  Don't think many other DAW's do it either.
 
The Klanghelm VU meter is one of the best and it is very cheap too.  You can use mono and stereo meters wherever you want.  You can change the ref level and the ballistics too.  AND right on the front directly under the meter is a nice built in trim control which allows +/- 20 dB of gain adjustment.  That is the place to  do it and the meter reflects the changes instantly.  It is actually faster than any channel trim control.
 
http://klanghelm.com/VUMT.php
 
Confusion reigns because many don't seem concerned or interested in rms and VU levels, everyone seems concerned with peak metering which actually does not tell you much at all.  Correct gain staging is near impossible with peak metering alone.
 
It's easy to meter incoming tracking levels. Calibrate your system at the chosen ref level eg -20. Calibrate any VU meter to the same ref level.  Playback alignment tones or set up signal generators accurately.  In track record arm and insert a VU meter plugin first in the FX bin. Monitor the incoming signal and have the track fader, buss and main faders at unity gain. Adjust your input preamp gain for 0 dB VU most of the time.  Very easy and straight forward. The meter will pin at +3 dB but there is 20 dB of headroom there should things take a sudden twist.

I don't mess with the gain unless I get stuff from a client that has a super low signal level. I see clip lights hit on plugs all the time yet I never actually HEAR anything clipping or sounding bad. I record all my stuff between -10 and my peaks at no hotter than -6dB. No one probably agrees with this and that's fine...but it works for me, I have plenty of signal and can always back things down if I need to. I've always recorded at that level and have never had a problem. I have done projects with complete gain staging and to be honest, I don't hear or notice any differences. But that's just me. :)
 
-Danny

I'v also never actually used the gain, before (always been at 0 )
I never had any real problems that would force me to use it.

It is also easy to " forget " that you have changed the gain, will not always notice it

I asked, because if there's a " common " way producers using gain as RULE in the audio world.

i guess as long the record of the track it is not peaking there's no need for gain usage !
other than if it going into the insert effect too HOT, then i can drop the gain to a suitable
level if needed !

I have never yet heard any FX...distort in my DAW, more or less regardless of
what the input volume was.