If it is a lap or throw, I don't like to use muslin because the entire quilt will be on display a lot of the time. It will get turned over, wrapped up in, etc. I want both sides to be pretty, and in my mind that means color!
I have seen some very great muslin backs though, that showcase the quilting. Guess it is each to his own, which is what is so appealing about quilting. I can do my own thing!