it depends on how you are quilting it...if you are hand quilting- piece it any way you want= if you are sending it to a long-arm quilter the seams really should be horizontal- when the quilt is loaded on the frame the seam will run evenly along the rollup bar---if it is vertical it builds up layer upon layer in the center-causing wavy=saggy sides.
if you are machine quilting it on your domestic machine i think it is your choice.