Does a flat ceiling provide the best sound?

Speakers are designed with an 8ft flat ceiling in mind.  Anything else requires adjustments and compromises

good info above - flat ceilings are much easier to deal with in terms of calculations and empirical treatments; the same is true for rectangular walls vs. trapezoids 

read more in the Master Handbook of Acoustics

no matter what, I predict you will wind up with some pads up in the corners of the ceiling & 2 walls

I agree with Erik. I have a 9' ceiling and  treated the corners with ACS tube traps, back and front walls with various diffusers, 1st reflection points with smaller tube traps and GIK diffisers. Room, soundstage, imaging, music is superb. But the reflection from ceiling is very noticeable to me now.
Had back surgery and am just waiting to get my balance fully recovered so I can install the ceiling panels recommended by Nick at GIK. Very good people and laser recommended by "Jim Smith; Get Better Sound" was/is an indispensable tool in a 107yr old house. Or for Speaker setup Period!!! 
I am an field engineer for 40yrs. Have plenty of steel tapes and am used to accurately measuring. The laser will put the tape to "shame" and make it (3) times easier.

Best to All on this Journey 

I have 16 ft ceilings (though flat) and it was a nightmare to adjust for good sound.

Great advice--interesting to hear that speakers are designed for ceiling heights of around eight feet--will have to keep that in mind.