Besides ceiling reflections (the floor will be just as bad if not carpeted) IMO the biggest issue is the height of the drivers. My experience is that center channel speaker placement works best when the speaker is either the same height as ear level at the seating position and/or the midrange and tweeters of the mains form a triangle with the center speaker where the center falls somewhere within the boundaries of the screen. Like the pictures in this post.
http://www.axiomaudio.com/boards/ubbthreads.php?ubb=showflat&Number=158274#Post158274As long as the center isn’t to high above the mains it doesn’t screw up multi-channel music and IMO actually improves movies because it better anchors the center image to the screen since many audio engineers blend some of the center channel information into the mains to even things out.
