I am going through the same decision because I am about 1 more evening away from drywalling the ceiling, and I need to figure out where the center speaker wire should go. After many questions and pointers from "audio experts" I was told that the center speaker above the screen works just as well as below the screen. Just make sure that the speaker is pointed downward directly towards the listening spot. Just like you would point the center upward just a bit if it was 2 feet off of the ground on a shelf.
After many discussions with my wife on the layout of things, we are going to put our center on the ceiling above the screen. Know this, though... The ceiling where the screen goes is actually about 8" lower than the rest of the ceiling due to the house's main ductwork being there and being framed around.
Hope that this helps...
