Backings for me are determined by the amount of fabric I have, and measurement of quilt. If I can do it all with one fabric, I will piece the back the way that uses the least amount of fabric-usually a crosswise seam for twins/doubles, and lengthwise seams for queen...and if I don't have enough of a fabric, but still want to use it, I will piece my back, if I can using leftover fabric from the front of the quilt, otherwise fabrics that coordinate. There is no wrong or right way-just whatever works best for you.