Just as a personal note, a few years ago I got a meter to measure the sound in my room.  I was surprised at how LOUD 85 db was!!  If that's low, then it makes me wonder how loud LOUD monitoring is!

85 dB SPL of pink noise is quite loud compared to music at the same level.  eg I use a Steely Dan CD track as a reference (only from the last two CD's, all digital affairs) and when I get it to push the meter just to 85 dB it sounds softer than pink noise.
 
85 dB SPL sounds nice to me but in mono on that small speaker I go well down to 70 dB SPL and under.  Check out 65 dB SPL which is a whopping 20 dB below 85 dB SPL!
 
I go up in 5-6 dB or so, 91 dB is a nice level as is 95 dB then I do a crank up to 100 -105 dB SPL for some serious bass and reverb listening tests but not for long!  It does sound bloody excellent but really we should not be up there for that long.  8 Hrs (eg one full day) at 85 dB SPL translates to a max of around 7 mins at 103 dB SPL.
(3 mins at 106 dB SPL!)
 
 
 
 
 

78 dB SPL is perfect for MY ears in MY room
 
Change any of those variables and everything else changes.

I think being consistent is very important, but so is monitoring at different levels. There are some things I mix expecting them to be played at lower levels and others at higher levels. I often have at least one target level in mind as I mix.

That said I mostly monitor at a consistent level and I don't use a meter to find it these days.

I feel having the meter around is a good thing.  The SPL level in your room can creep up over time.  Before you know it you are at 90 dB instead of 85.  The meter keeps you on track.
 
 

The BBC Academy have a good set of articles on audio monitors here
 
Jerry

*Another* thing I don't do properly.  Haha!  I'm pretty sure my mix level is generally much lower than 85 dB.  
 
Thanks!
 
Donny

Wow...I just checked...I'm typically around 70 dB C weighted...guess I'll be doing this when no one is home.