Rear speakers in a surround sound system

So I currently run a 5.1 system using Axiom speakers, but my Cambridge Audio receiver is a 7.1. I am considering getting the rear speakers, and am wondering this....
I have QS8's on me sides, should I use the same in back, or would the QS4's or even just some M2 bookshelf speakers be fine? I ask because for one, I don't think most movies even are made in 7.1, and two don't think of what sound does, will be that noticeable anyway. You thoughts?