I piece backings a lot. I try to use the leftover fabric from the front. If it will look too patchy, I use something else that will go with the top. I buy fabric for backing when I see a good sale of a tone on tone. I'll buy 5 yards or more. Then, if the quilt top has greens, I will use a green tone on tone fabric I have purchased on sale. I then piece that so the back looks all alike. I usually do 1/2 inch seams and press them open.