In stereo listening, the weakest pair will be the top level of your system. I think the same idea holds in multichannel systems. Weakest link and all that.
Rear speakers, generally of lesser quality?
Necessary for Rear speakers to be just as good as the main speakers? I had some extra main speakers put them in the back of the room. I think they sound a lot better than smaller rear speakers i used in the past. There is a lot of information coming out of the rear speakers via 5.1 set up. Why go cheap?