I understand from seasoned quilters that using printed fabrics for backings will hide any minor mistakes you might make while quilting. Solid color, muslin, etc tends to show every stitch.
If you're short of fabric to complete the backing, many quilters work in extra fabric from the quilt top fabrics. You're the only one that knows this didn't start out as an artistic endeavor.
I currently have waaaaaaay to much calico print in my stash, so I'm currently doing a simple patchwork of these and using it as backing. When I get tired of that, I start using something else LOL
I guess the bottom line is whatever works for you "it's a good thing"