I put my QS8's on my back wall in the position a normal sbl & sbr would sit because I had intentions of moving to 7.1 fairly soon, & I didn't want to have to move the side surrounds (in 5.1 they are better off behind the seating area, while in 7.1 they recommend them to be directly on either side) once I bought the other two QS8's. But it has sounded so incredible I've yet to even upgrade to 7.1, so don't worry about putting them on the rear wall. Even though that might not be their ideal position, they still sound remarkable back there!

My Stuff :

M80's
QS8's
VP150
EP800
Denon 4802
Emotiva XPA-3
Samsung BD-P3600
Sharp 65 Inch Aquos LCD