The human ear hears directionality in the horizontal plane and not so well vertically. The more you can place the front speakers especially, firing in the horizontal plane at seated ear level, the better the surround sound and stereo imaging. The side and rear channels often put out an ambient or background field and only occasionally a directional sound. They are usually placed about six feet high on the wall firing horizontal. If you have to raise them to ceiling height angled down, it is probably not going to alter the sound drastically and IMHO should not be a problem.
The center channel function is to anchor the voices especially, but probably 60 percent of the total video soundtrack, to the screen. It is adviseable to put the center channel behind the screen if it is perforated for sound, or just above angled down or just below angled up. If center speaker placement does not permit this, you may have an alternative by not using a center channel and using the front left and front right in stereo mode with the speakers near the sides of the screen. If the speakers are separated to far, then there could be an audio hole in the center without a center speaker. The center channel is considered to be the most important to the surround sound system along with maybe a good low extension subwoofer. Good left and right mains are considered most important to music stereo imaging.
