That's interesting to me because I don't think I've ever dimed any EQ knob/slider. I've certainly un-dimed extreme lows or extreme top end though, as well as to notch out an occasional offending feedback/booming frequency.
I'm not afraid to boost that much, but I guess I usually like the inherent voicing of the amps/cabs I play with enough that a change that radical wouldn't appeal to me.
As for gain, I'm a fairly clean player so enough break-up that I get warmth through natural compression and some added harmonics is about all I go for. I always wondered if anyone used that last 30-40% I never get to on a gain control....now I know!