Mike, when you have a 7.1 speaker setup the side surrounds are ideally directly to the side of the primary listening position, where they provide the best side surround effect. They don't have to be slightly behind, as is often suggested for a 5.1 setup, because there're actual speakers in back doing the back surround, so no help is needed from the side speakers. The position you propose would give the back row the optimum side effect, but it would still be good for the front with QSs; do what you have to do.
The back speakers should be vertical if at all possible for smoother and wider horizontal dispersion and positioned about 6-7' up. Spread them widely enough so that the seats can get a definite left rear and right rear effect(i.e. each listener has a speaker behind him to both his left and his right).
