I'm not sure which wall you're talking about. Usually when we say 'back wall' we mean the wall behind you as you face the speakers... but from your post I'm inferring that you mean the wall behind the speakers, that you face while you're mixing.
I'll try to advise on both.
With free-standing speakers, the majority of the sound that will be going out the back of the speakers and reflecting off the FRONT wall will be low-end sound. Making that wall reflective to the highs isn't bad, but try to absorb as much of the lows as you can there.
With any speakers, then wall behind the mixing position should absorb as much as possible, over the entire sonic range. The idea is that sound should come out of the speakers, go past you to the back of the room, and not come back.
Anyway, if you have other ?s about the room, like laying the walls out of parallel or how to build absorbers, there's plenty of that in other posts...
Good luck!
Kase
www.minemusic.net