FWIW....
 
after i get my individual track faders sitting where they need, for a basic balanced mix, 
i break everything up into sub busses, that all go to master...
 
i find that after eq'ing and adding sub buss compression and limiting,
i always need to reset, say, the 'guitars' sub buss..
 
so, basically, i end up putting automation on almost all my sub busses,
the ones that have more than a couple of tracks into them,
and i mix with my sub busses..
 
point is, i very often end up bringing my sub busses down well below 0, which is where they all start out when i'm doing basic first level checks.
 
the thing you have to watch out for,
is HOW the gain structure changes when you are pushing, say, a limiter across an overheads sub buss, when you bring the sub buss down, your level into it stays the same....
 
but, if you drop the levels of your channels going INTO the sub buss,
the level into the limiter changes, which changes all your dynamics settings, and you have to start over.
 
so, there is alway push and pull,
but the main take away is, give yourself PLENTY of headroom.
 
there is no downside to having tons of headroom.
 

I think it is important to get the balance right on each buss and yes not move track levels going into buses after you get that mix right.  My buss level faders are often still at 0 dB or unity but the rms level on a buss is often below my ref level.  eg  If I am working at -14 all the way through a project, then buss levels will be 3 to 4 dB down on that but then the buses usually all sum back to a balanced mix sitting around -14 again which is what you want.  The buses in my projects have slightly higher headroom than the tracks themselves.  They are often carrying a more complex signal so they might benefit from some slight extra headroom.
 
Headroom is great. The sound quality is very consistent over a wide range of digital ref rms levels these days. So working down at -14 or -20 is a breeze.  The sound there is pristine and transient and dynamic.  I rarely see overs anywhere when everything is running nice.
 
The SPL level in your studio is an important level to set and get right.  That is where you get the volume and power from your system.  My system sounds devastating when I turn it up loud and I can go loud anytime and it is good to listen to a mix loud for a while.  It can be very revealing.  I also really like listening way down soft in mono on a small mono speaker.
 
Gain staging is important and I never get bored talking about it.  After a while every project you do ends up with levels being perfect everywhere.  Tracks, buses and the final master buss.  It is something that existed in the analog days and the whole concept can be lifted across into digital.  It sounds better there and there is no noise now.  Transients are better.  As a drummer I can hear it big time.
 
I find VU meters really help me achieve great gain staging every time.  Peak metering is important now.  Work with both at once.