it really depends on how you plan on finishing it- if sending to a long-arm quilter the seam will (hopefully) be horizontal--across the quilt- when it is loaded on the roller bars the seam will lay flat along the bar- if your piece is vertical you have layer after layer of bulk building in the center- and baggy edges
if you are hand quilting it you can do it any way you want-
or if you are using a domestic machine it probably does not matter- you can piece it any way that works...but it does really matter for having it long-armed.