I don't know if you have seen my pictures of my setup, but my QS4's are right on the back wall, and if I sit on my bed and watch a movie, they are 4 feet to the left and right of me, and about a foot above my head.
Sometimes, I think a QS4 is above my head, they do that great of a job, and that setup works great. I would definately keep them on the back wall. For me, I really like my surrounds behind me, makes everything sound more enveloping.
Another thing to consider is keeping the surrounds as low as possible, so they are very close to the height of the listener's ears. Having the surrounds high seems to make the surrounds more "ok, there is a speaker there, and a speaker there" sort of thing. You might be able to get away with that better on the side walls, on the back walls it might be too close to some of the viewer's ears at some spots.
I am not sure if you are constructing and planning to run the wires to the specific surround spots, but I would try moving the QS8's around to see what sounds best. That's the only way you will really know. Try screwing the QS8 brackets onto a piece of plywood, screw the piece of plywood to a 2x4, and you have a portable QS8! Move it around and see how it sounds in each spot.
Good luck setting up your new HT!
