Lots of people piece the same fabric to make it the right size. But you do have an opportunity to jazz it up a bit by using the border fabric you DIDN'T use for borders and add that to the back! Or an extra block or two with strips of the border or back fabric to include the blocks in a pleasing manner. I agree, I don't do a strip right down the middle. I offset to one side. I just finished a very colorful quilt and used a jelly roll, sewed into strips and set on the diagonal across the back. Was a trick to cut the large backing pieces on that same diagonal, but I managed, and it looks awesome! At the long arm quilters right now or I would post a pic! Just have fun, no law that the back has to be 1 piece, 3 pieces or even all the same. Some folks make scrappy backs, others actually piece two tops and make it totally reversible with 2 different sides that are a quilt in themselves!