We have a "spinner" at our gym that brings a wrench and tilts his seat up (leaving it like that after he's done--of course). He got a hold of my favorite #8 bike while I was out of town and I didn't notice--resulting in a miserable hour spin and sore for a week afterward.

The seat should be flat--not nose up or down (nose down can help with your core but can also slide you forward if your core is weak).

When picking a bike, look and see if all the seats are the same. Speak to the instructor and others in the class. Our gym had a new tech that tilted all the seats up "because that was were they were supposed to be" and there was a revolt.

Sit up if you have too. I have a bad back and some classes are better than others. Stand if you need a break.