What he said. Side walls or ends of the couch. Try both if you can and make sure you run room correction or make the proper adjustments in the AVR.
In ceilings are difficult to get sounding right in a vaulted ceiling. If you decide to put Atmos speakers up there, put them slightly farther out from center than you normall would.
Enjoy the B&W’s. New 600 series in nice. Also, since you’re only doing 5.1, don’t forget you can probably reassign rear channels to bi-amp the front!