I usually end up standing out of necessity. I stood the whole 40+ minutes up the backside of Lookout mountain two years ago at the 3 state 3 mountain century. I was still only going about 2.5 mph and out of gearing.
I find I can get a better rhythm with standing and usually alternate with 40 pedal strokes per leg standing and then sit until I'm ready to stand again if the hill requires so.