I haven't come across any definitive answer to whether the surrounds in a 5.1 setup should be *facing* forward, sideways, or toward the listener. Is it a "whatever sounds best" thing?

My room is about 15x20, with the TV and mains toward one of the 12' ends and the seating at the other 12' end. There's a wide-ish doorless opening on one 20' side and tall/wide glass on the other. The ceiling has a 12' barrel vault over the rear 3/4. I'm trying to figure out if I should mount QS8's in the small part of the 20' wall facing sideways, on the 12' back wall facing forward, or in the rear corners facing the seating. I can move the seating forward a bit, but probably not more than 2-3'.