Sorry John but Toni Grimmani and Lucas Sound disagree!
The rears should always be placed close to the ceiling and never at the ear level. Rears are supposed to be diffusive even in a direct radiating system. The idea of surround sound is to mimic what you hear in reality, as you move further and further away from a sound source you loose exact placement and instead hear the sound as emanating from a destination location. Most surround sound effects are trying to duplicate spacial cues to mimic the reverb times of large spaces to pull you into the movie, or to recreate pans from side to side or front to back.
If you can pin point where the sound is coming from you are defeating the concept. Direction not location is what we are aiming for. In fact most professional theater designer still prefer di-poles radiators for the side locations whenever possible.