Jeff, regardless of where the surrounds in a 5.1 setup are placed, they're side surrounds and play the material in those channels, not the material in the back surround channels of a 7.1 receiver. They should be placed primarily to the side of the listening position, although being maybe 2' or so farther back than directly to the side can help add a little "phantom" back surround effect in the absence of back surround 7.1 speakers. Note that if the listening position is almost up against the wall, mounting the side surrounds on the back wall but spread widely, almost to the side walls, produces a similar effect.
