From the Axiom FAQ:
Where should I place my surround speakers?
Ideally, surround speakers should be either wall-mounted or stand-mounted to either side of the couch or primary listening area, from one to six feet higher than seated listeners' ears, and slightly forward or rearwards of the listening area if you wish. Axiom's Quadpolar surrounds are especially versatile and can yield excellent performance with stand or wall-mounting to the rear as well as at side locations. (see our dedicated stands for the QS Surround speakers in the Store). Rear center speakers for 6.1 and 7.1 installations are intended to go on the rear wall at about the same height as the surround speakers.
Keep in mind that this is only a guideline. Most of us, in the real world, have to adjust placement to fit our rooms. The closer to this you can get, the better, but don't agonize over it if you can't get exactly these angles.
