I have done both. For smaller quilts, I've usually got enough of one of the fabrics that's in the front to make a backing out of. For big bed sized quilts, I'll buy a wide backing that somewhat goes with the front. And muslin is a real dream to quilt with/on!
I guess it depends on what you like on your quilt & whether you want to make a backing or buy one already big enough. It's your quilt - do what you want with it!
Linda