Since the backing is as important as the quilt it is very important that the fabrics be a complete match. Because I don't always have the perfect eye for this kind of thing I frequently will use blenders and be safe. Then there are the times that I purchase the backing when I buy the primary pieces for the quilt so it will at least match one piece in the quilt then I know it will work. Now that I am working from my stash most of the time I try to pull a fabric that I can use small amounts of a few different pieces in the quilt top and sew together larger pieces of the pieced part to complete the backing. I like to make backs that look like overgrown crazy quilts for the back this way. It seems to work very well.