I stand every 5 min for 30 seconds while on the trainer. I do it to give my butt a rest and to break up the boring intervals. If I start a 20 min interval I'll stand at 2:30, 7:30, 12:30 and 17:30. I normally ride around 90RPM and shift up a few gears and pedal at the same speed but a cadence of 70RPM while standing.