I assume you are asking just from a sonic point of view. Also, I assume the quality of power is the same in all parts of the room. In this case you just have cable length and isolation to worry about. Best cable length is between the speakers, but it is the worst isolation. Best isolation is behind the listening position, but it is the worst cable length. Most people recommend a side wall placement to balance cable length and isolation effects. I have gone with between the speakers for best cable length and dealt with isolation by getting really good stands.
