A few quilters have asked lately if it's ok to send along a pieced backing. As long as the backing is still 3 or more inches bigger than the top on all sides, then it's fine. Sometimes a creative backing can be a way to use up some extra blocks & add some interest to an otherwise plain backing.
If you have some leftover pieces or blocks, piece them together & insert them between two sections of yardage.
The only caution I have is that I can center the backing to a point, but not always perfectly. If you make the backing asymetrical, then it will be easier for me to work with. Here's a great example of a backing on one of my baby quilts. The inserted strip is off to the side.
When the backing needs to be perfectly centered, it can be tricky, as it's hard to tell exactly how much the top will draw up when quilted. It may end up off to one side or the other by an inch or two. So, just keep that in mind when piecing your backings.