I usually use a printed fabric for backing but i may start using muslin on the backs if it's for charity.
I feel that if someone is paying me for a quilt that they probably want a printed back also.
BUT if you buy quilts from a store I don't think any of them come with printed back fabric.
I made my daughter & SIL a quilt for their wedding gift and used a cream colored sheet for the back and it really looked nice instead of a print of some sort....I have a brown print batik on my bed quilt and now I wish it was a plain muslin.