I'm sure others will have better reasons and better explanations, but I've always just run what "feels" right (I'm normally a stealth guy, so my experience with stands is somewhat limited anyways). I try and look at the room as though the tops of people's heads are the floor, and the ceiling is the ceiling, then fly just about in the middle. If the crowd is particularly chatty or anything (clapping above their heads, etc), I'll fly higher, working on the belief that it's harder for me (personally) to be disappointed by a recording which has a weird audio property such as a ceiling three feet above the mics than to be disappointed by some jerk screaming all over the tape. It's much much much harder for me to even notice weird audio anomalies like ceilings, walls, etc. I'd say so long as your not hugging a barrier (translation: don't run two inches below the ceiling-- give yourself a pad of air between the mics and the ceiling in this case) you should be fine. Test at home if you want though, you won't need people talking, just turn on some loud music and fly high, fly low, fly middle, fly all over the place.... then just review what sound you like and when the walls / ceilings start affecting your mics.
Hope it helps
-b