No. Don't use 5.1. Most audio (99%) was mixed with stereo in mind. Adding more speakers to the mix ruins this original recording, And adds nothing to the experience. So why waste the money? It's a farce I tell you!
I agree with you on that, and you should probably switch to stereo on the AVR, but 5.1 sounds better and louder on my AVR than just using 2 speakers.
As for the actual use of 5.1, I kind of enjoyed GTA V in this mode.