I like the backing to have some connection to the front of the quilt so I use yardage that I piece together. It doesn't have to be fabric that was in the front, just something colorwise that works. For baby quilts I like to use polka dot fabric where some of the dots mimic the colors on the front. When I have a large quilt quilted by a longarm quilter I try to use wide backing so there are no seams in the back. I buy neutral colored wider backing on my trips to Lancaster PA where I can purchase it cheaper than locally.
I like the look of backing pieced with different fabrics and/or blocks but for gift quilts I make the backing all one fabric because non quilters don't appreciate the mixed backing like we quilters do.