I started out with printed backings to match the fronts as I've only made large lap quilts (some are actually twin size, but will be used as lap quilts.) Then I came up short of fabric and used some braids to make it bigger. Now I like stacked coins or something to break up the expanse of sameness. I have even made a full quilt for the back because I didn't like the front - now I do, so that daughter got a bonus. I use thread that may match something in both sides. My FMQ machine (Singer 301) doesn't show anything so I can have red varigated on the top and yellow on the back. I did like a blue varigated on the back of a dotted dark blue back. It was a QOV and the quilted stars showed up that way. In others words, don't worry too much. Once it's stitched, it all goes together (within reason, of course).
I've even put pictures on the back when I ran out of space on the front. I was concerned about the quilting, but finally decided the front was the feature and quilted like I needed to, though I did avoid the girls' eyes.