I go with what will annoys me the least. I sometimes buy the 108 backing just so I don't have a seam. If I decide to piece a back, I usually put the seam crosswise. It really bugs me if the seam is lengthwise and it isn't perfectly straight. If you decide to put it up the middle lengthwise, try to center the quilt top with pins and basting before quilting. I do like the backings that are pieced with lots of pieces and leftover blocks because it makes the quilt reversable. Backing construction is a personal choice, there are no right or wrong ways unless it is being judged for competition. Some competitions have rules about backing construction (quilt police). :-D