In a 5.x.x you have SURROUNDS not REARS. Your surround speakers should be to the side at a 90-110 degree angle.
In a 7.x.x you gain Surround Backs commonly called REARS. These should be placed at a 135-150 degree angle....
Going over REARS again
The #1 most miss-construed point we see in this Subreddit is the definition of surrounds. Let me say it clearly: SURROUNDS IN A 5.1 SYSTEM DOES NOT MEAN SPEAKERS BEHIND YOU!!! It means speakers on your SIDES. 90 degrees-110 degrees to your sides or slightly to the rear of your main listening position slightly above ear level, pointed INWARD, not toward the TV!
Speakers behind you pointed forward toward the front stage are the REAR SURROUNDS in a 7.1 setup. Period, the end.
Q: My couch is against the wall should I add surrounds?
A: In a 5.1 setup? Sure. They just go on your sides. But 7.1? No. Don't try to cram a 7.1 setup in a space that isn't suitable. You're just wasting your time and you won't get any separation in the sound stage. Separation is important! Same goes with Atmos. There is no shame in a solid 3.1 system either.