When I first started mixing at my new job I was a per diem contractor who was just there for rehearsals and services. I was walking into a situation that had been pretty well set up by a pro but had been handled exclusively by volunteer operators for quite some time. There was a lot of stuff that needed touching up. Church sound systems are kind of like stalagmite formations in caves after their initial setup. Lots of things get done "just to get things going" and wind up staying like that.
The thing that really surprised me was that the main faders were set at about -20. I went ahead and mixed with them like that for a while but the first chance I got I went in and zeroed the board to start fresh with them up at unity like they should be. Right away there was an audible difference. Here's why.
The problem would occur when the band got too loud. Someone would complain and the guy mixing would respond by pulling the main faders down instead of re-mixing the band. While it works, it doesn't really do you any favors because what's balanced with the mains in one spot might not be at another spot. At any rate, the following week at rehearsal the gains would all get bumped up to where the op thought was a good sounding spot and the whole process would repeat.
Eventually you wind up with the mains at -20 but the gain has been made up on every input on the board (thirty or so out of a possible forty-eight in this case). Many of you will be familiar with the practice of setting channel faders at unity and bringing up the gain until you've got what you want. You do that so your gain structure is optimal and you can mix in the sweet spot of the fader where you have the finest control. With thirty channels 20 dB hotter than they needed to be the system was hissy when it was quiescent and all the inputs were squashed from overdriving the mic pres when the band was rocking.
So it's pretty important to understand your gain structure and get it right, or at least as right as you can most of the time. Remember, rules are rules and you should never break them... except when you decide to. Even on a small mixer it makes good sense to set things up right because small, inexpensive mixers are usually not very quiet or tolerant of abuse to begin with. If you're just starting out, spend some time with a small system or with just a couple channels on a bigger one if you have access (you have both if you go to my church, hit me up) and learn how the different gain stages behave. You can also rely on the word of other pros like the descriptions Karl Maciag gave us a few posts ago about different brands of mixer and where they like to be set.