The OP doesn't say what levels the recorded tracks come in at (or maybe I missed it). If the meter on a track is showing -0.2 dB with trim and fader at 0.0 that's too strong. But maybe he's got the fader set below 0.0...?
The way I would say to think of the trim vs. faders is you use the trim to set up to do a mix, like setting a table before you eat. Pre-mix you would set trims so the signals are optimal for each track - remember faders have greater resolution closer to 0.0 dB so you want to be in upper segment for greatest fine tune control - other things you do before starting a mix would be trimming the silence away form clips, basic housekeeping stuff like that, setting up send fx etc.
Then when you mix, you mix for expressiveness, including automation moves. The control surfaces are intended to use for mixing. Although with the Mackie you could I think assign an assignable knob for trim but since this parameter doesn't need to be "ridden" there's not much point really.
In a perfect world if your tracks all get captured at perfect gain levels than you wouldn't need trim but rarely does this happen, usually ther are at least small adjustments to give an optimal starting point.