What I am referring to Chuck, is your gain staging process, which is preferably done at the start of the mix session. Once done at the beginning of the mix session, it (gain staging)can be left alone.
We are not trying to mix with gain staging, just trying to get our levels and faders at an optimal point to start our mix.
Here is an example of how I gain stage , and I'll use an example of a client sending me a mix, which I have no control over what was recorded and how hot or how low they recorded their tracks (given that none are clipping):
You can actually turn your monitors down for this process.
 
1)Pull all your Faders up to 0db including Master Fader. 
 
2)Now play your tracks and go through each track one by one (yes tedious process as another forum member stated in a earlier thread) and preferably go to the loudest part of each track (look at the waveform to save time) and adjust your gain knob for each track(faders still at 0db untouched) so that your peak meter on each track is hitting around -18db to -15db (we are ignoring the Master Fader meter "at this stage" of the process.
 
3) Now that that part is done you can start your mix, it takes some time trial and error to learn where to start a certain instruments  peak at to get used to how and where the sum of your master Fader meter is going to peak when all tracks are in, this part is not exact science due to variations in material from project to project, but you have to get used to having a base instrument to start with. 
So lets say my gain staging that I just described is done and I haven't done any actual mixing yet (only gain staged my project).
I will drop all the Faders except for the Master Fader (at 0db) and start to do a static mix balance. The next step just takes practice, and this is only my particular method, there are many different approaches so i'm just giving you an example. Once I start my static mix, I am make my initial Fader up move  on the kick and try to get that to peak at about -14 / -15db. Now I can ignore my meters for a "little bit" and finish my static mix of my levels. Once I am done with this I start fine tuning and my end result ends up being -6 to -10 at the Master Fader when my mixing is done.
Takes practice and this is only one method, but the important part is the initial gain staging process. Makes things so much smoother and improves the end result.
 
On a side note: If you are recording the material yourself, the optimum process is to initially record your material to hit -18db to -15db.
As far as whats optimal for your plugins, its not about a constant -18 to 15db, once you gain stage properly that is taken care of no matter where you move your Fader to, since you have got your input level optimized.
 
The beauty of the DAW age is that you can experiment (if you are afforded the time).