I think the only time it could be a problem is if you are sending it out to a longarm quilter. From some of the posts I've read in the past, some of them may prefer a one-two piece back.
If you are quilting it yourself, anything goes! Some of the backs are as pretty as the front!
EDITED: I should add that if you plan to send it out for quilting, it's probably wise to speak with the quilter before you piece the back to be sure they can accommodate you.