Others will probably prove me wrong but, to my ears, I'd say to stay out of the ceiling.
My neighbor has a 10,000 sq/ft home and was built with only minor concern to cost (slapped it in the middle of 40 acres and has an elevator in it)
I was over there one day and the wife just GLEEMED with smiles telling me "you have to come into the living room and hear our surround sound"
Having been there before I feared what I was about to witness.
Turns out their ENTIRE 'surround' system is in their ceiling. Their ceiling is about 12' high. As I sat there, tv in front of me, I felt like the Voice of God was coming from above telling me about the local weather.
Granted, they have ALL of their speakers in the ceiling and that is not what you are asking.
I personally have no desire to go beyond 5 speakers....where do we stop for sound effects? 43,492.1 ?
If I was considering in ceiling speakers, I'd probably first go investigate in wall speakers so I could perhaps bring them down to more in line with my ears. That said, if you must/want to use ceiling, there are some companies that make "directional" (for lack of better word) ceiling speakers that you can aim a little bit.