Believe it or not, surround speaker placement is still hotly debated in the industry.
This is scary because the assumptions used will dictate monitor placement in the mixing studio and hence what the mixing engineer hears. If this conflicts with your surround placement you may not hear what the engineer intended.
Some like Tom Holman (THX chief) and Wendy Carlos strongly advocate about 90 to 110 degree placement. These two are very vocal and critical of other placements.
Others like the Grammy Arts & Sciences wing strongly advocate more rearward placement.
In my limited experiments, I like about 90-110 degrees seems best, which is the ITU recommendation.
A big factor is your chair or sofa design. If you have a recliner or anything behind your head that blocks sound from the rear, you definitely don't want rear surround speaker placement.
In general the QS4/QS8 are not as placement critical as other speakers. I think back wall mounting can work out OK. You have to be practical, and balance aesthetics and acoustics.
Wendy Carlos surround discussion
Dolby recommendations
Tom Holman (THX chief) recommendations
Grammy Arts & Sciences recommendations (PDF) 
