When I need to widen my quilt back I prefer to place the inserted piece or seam on a diagonal. If it were dead vertical it would become bulky as it was rolled up on the longarm roller and would distort the quilt back. Also, trying to get the seam in the center is an exercise in frustration. If it becomes just a little off while quilting, it will always look...a little off. So I deliberately make it off-center.
Some may say that the seam will get more wear when the quilt is folded in the middle. I try to fold my quilts in a different way every time, isn't that what has been recommended by experts? And folding a quilt in the same way turns the fold lines into permanent creases and can cause fabric damage over time.
Here's a pic of back art on a quilt I entered in the 7 Sisters show last month. Sorry the pic is so big.