All of the cautions others have listed should be kept in mind if you're new to spinning. But, like kit says, if you feel like doing something, and it doesn't hurt, go ahead. I used to be into the "good instructor" mindset. Now I'm in the "as long as the instructor doesn't talk too much" mindset and follow my own needs on any particular day.

If the instructor's routine fits my general needs, I'll follow it. But if I need something different that day, or if I'd rather follow the tempo of the music, that's what I go with.

Our gym has Kaiser spin bikes that use magnetic resistance. They are waaaay better than traditional spin bikes that use felt pads. As a recreational rider, I find that spin classes on the Kaisers are very effective in developing useful cycling power.