If your room will allow it, the ideal placement would probably be on the side walls with the null pointed towards your main listening spot. On the back wall is certainly acceptable, so long as they're a fair distance apart (ideally in this circumstance, there would be at least one side wall or bookcase for sound to bounce off of).
What you could do is recruit a friend or two and experiment with placement while playing some DVDs that make good use of the surrounds (Lord of the Rings, Attack of the Clones, Gladiator, etc). If you don't like the way the speakers sound behind the couch, buy a pair of stands.
Overall, QS speakers are VERY forgiving of suboptimal placement, so don't lose too much sleep over it.
