ceiling is fine, they fire so seldom it really doesn't make much of a difference. If you play surround SACD's or DVD-audio, then it can make a difference, but for HT, some ceiling speakers are fine.
Speaker placement for the rear HT
Thiel CS 2.3s up front, small moniters for the rear in a 5.1 channel HT system. Some people place speakers mounted from ceiling, some from side or rear walls. I know every room is different. Any consensus on most rear HT speakers