to ensure maximum quality of mixing, and not let unnecessary electrical distortion and hiss get in there, it's always good to have it high at beginning of the sound chain and lower at the end, with the reserve of common sense off course , this means :

to not get anything over loud in the mix first off; it's good that when you do your tryouts to keep the gain centered or whichever is the closest to a decent level, this all depends on the mixing table ....

like mentionned earlier it's good to keep the faders up, now my addition to "ajusting" the system to your use, would be to play loud, keep the masters at a 3/4 level ( depends on the db rate of the master ) , and leave it at 3/4 giving yourself margin to higher the volume during key tracks or specific moments as you wish.

Concerning making sure you don't blow out the system I suggest checking out your setup with, like I said, mid - level decent gains, high faders, 3/4 master, and if that starts distorting either lower gains ( not fader ) or master, or better yet volume control on the amps because if you do so you'll be sure not to screw up distorting by putting things on to loud on your table ......

It's a precarious situation and it's hard to give you exact details on what to do cause it depends on so many parameters that can't all be treated equally cause different from one system to another.