I just pieced a backing for a California King quilt. Since I also also longarm as a business, I know that most of the seams need to run across the quilt...those that run from top to bottom build up on the rollers and can cause uneven tension on the backing which will result in pleats back there. I used some blocks from the front that I didn't use there, and large pieces of material I had left over...nothing smaller then a fat quarter. When you do piece a backing, use 1/2" seams, press them open and back-stitch on both ends so it stays together. If you are having it quilted by a longarm quilter, it needs to be 8" longer and wider then your quilt top.
I have also done a large star in the center of the backing that replicated the blocks on the front...that one was a pain! Hard to make sure it was centered on the frame to meet the center of the quilt top. But it is now a lovely double-sided quilt!