This risk is the reason I always used pieced blocks in the MIDDLE of the backing rather than on the edges.
It is very hard to keep a got-to-fit-perfectly backing fabric where it needs to be after quilting the sandwich. Allowing extra fabric at fall extensively along the edges of the back of your quilt will also give you room to trim and square up your quilt after it is quilted. It's pretty hard to tell if a backing does not perfectly "fit" or match the front of the quilt because you really can't see both at the same time.
Jan in VA