Personally, I'd rather have the speaker above the TV and angled downward. Otherwise, the sound from a speaker below can be blocked by furniture (like a table), or even the first row of seats (assuming there is a second row of seats). Having said that, you also don't want your centre to be TOO high up, even if it is angled downward to you. The tweeter for my centre speaker is roughly 16 inches above ear level, angled downward, and it sounds pretty perfect.
Best bet is to experiment with different positions first. But I still think above the TV is better than below, depending on how everything is setup.
cheers,
supervij
EDIT: Here's one way of looking at it. Place an imaginary horizontal line through the middle of your screen. When seated, if your eye level is above this line, place the speaker above it. If your seated eye level is below this line, place the centre below the screen.