I have a 7.2 system. As I upgraded my Front L/R, I moved the old speakers back to surround duty. Ended up w/ 4x surround speakers @ $1k each. I like action movies with a lot of dynamic sound. Maybe, possibly, I think, occasionally I can hear a stronger effect with the surround towers. Maybe. I'm not sure.
Of course, at first I said "Wow... what great surrounds!". But after the initial euphoria, (that comes w/ any home theater upgrade), wore off, I can't really say for sure they make any serious difference.
My suggestion is to focus on your Left/Center/Right and subwoofer(s) first. Once you have them where you want, and still find you have some extra $ to spend, you can work on the surrounds.