I also would go for the QS8's ceiling mounted for optimum placement but if it is not possible nor desired, then mounting them on the back wall would work as well, the Q's are very forgiving and will fill the back of a room up with sounds very easily.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70