TV over fireplace? If so, then get yourself a Mantlemount (or similar) and attach the center channel to the bottom of the mount/TV. This layout solves many problems. First, it moves the TV down to a reasonable level, so you do not have neck strain from constantly looking up. Second, the center channel speaker will be more in line (laterally) with the front L&R (having the center in the ceiling, with the L&R at ear level will be problematic, as the sound will fluctuate vertically as it pans from left to right). Lastly, you want your front three to be as similar as possible, and floorstanding vs in-ceiling are about as different as it gets in sound character.