If it's a lap quilt, throw quilt, or bed quilt, I try to be creative with the backing. I usually have one fabric that I'll use. I haven't pieced any together yet, but I'm sure I will in the future.
On the other hand, for wall hangings, table toppers, etc. I always use muslin. Unless someone turns the table topper over, no one is going to be gazing on the quilting stitches on the back. That way I save good fabric for another use.