If you mount side and rear speakers on the ceiling, and but speakers that are fairly flat, it becomes a non-issue.
Take a look at something like the Vandersteen VSM-1, for instance.
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?