No, Diffuse in the back is fine too especially if you are only going with one speaker.

I have 4 QS8s and was at first disappointed at how little sound seemed to come out of the back speakers...... until I turned them off. I then really noticed that the little they do play does pull you in better.

They really do help surround you with sound and the few times something does happen directly behind you in the movie, It is great!