I wouldn't worry too much about imaging unless it's in front of you. As long as there's enough distance between two speakers behind you for you to be able to tell if a sound is coming more from the right or left, you'll be fine.
About your last paragraph, I'm not sure it would be a waste of material to wire for 7.1 now. If you don't do it and want to upgrade later, you'll think it's a waste of material to have to cut and re-patch your drywall. Also, if you wire both rear locations now, you'll be able to experiment and choose the location that sounds best to you.
