Very generally, I think you want your mics as low as you can "get away with" -- meaning, that most closely approximates what your ears hear in the crowd (since the show is usually mixed by someone whose ears are at roughly the same height) but avoids "too much" crowd noise (which varies by crowd, type of music, etc.).
Which is to say, there's no hard and fast rule, but you want to avoid things like being too close to the ceiling, or you'll end up with a bunch of unwanted reflections. And of course you have to make judgments about how much crowd noise you'll tolerate, basing that judgment on who is actually around you at the time.
This is also a reason I favor balcony clamps whenever possible... getting the mics out away from the crowd tends to be a big help in crowd noise rejection, if that's an option.