I did some experiments when I had my setup in a small bedroom and found that when my seat was right up against the back wall it was hard to tell the difference and may actually have sounded a little better with them on the back wall rather than to the sides facing in.
One experiment you might consider to help out your surround sound for dedicated movie watching is to pull the couch out a foot or so off the back wall while watching a movie. Even one or two feet can really improve how the surround sound field envelopes you. If that’s possible and it helps buy a set of furniture sliders to make it easy to pull the couch out and push it back in when not doing serious movie watching. Worked for me.
Cheers,
Dean
