My personal experiences with 5.1 and 7.1 are that the 7.1 mixes aren't a big deal but if you have a big room it's just nice to have more surround speakers to fill in the space. I just have the AVR use DPIIx or NEO6 convert the 5.1 to 7.1 for me.
Here is an early diagram for my HT that I just completed. I ended up putting QS8s on the side surrounds and QS4's in the rear. Because of how far apart the side surrounds are I'm glad I did it. Now that said I've had smaller HT rooms and I believe the two extra speakers in back don't add much there.