My backs tend to be pieced out of sheer necessity! I like to use a light neutral fabric on the backs, but often that drawer is quite empty! I usually take bits of the fabric from the top and do a simple pattern on the back, usually a large irish-chain or nine-patch, or even a small sampler. If the quilting is very detailed on the front, I will make it a very large scale pattern so the quilting doesn't get hidden or break down and go with a solid back.
I like the variety and interest of a pieced back.
Shelly