visit John Flinn's web site- he has a great free tutorial on piecing a back that saves fabric- and disperses the bulk so it lays nice and straight.
and it does depend on how the quilt will be quilted- are you hand quilting it? or, are you quilting it yourself on your domestic machine? or,
are you planning on having it long-arm quilted?
if you are planning on having it long-arm quilted it is best to have your seams horizontal- not vertical-
the quilt is loaded on rollers- when the seam is horizontal it will lay nice and straight along a roller- if the seam is vertical it builds up bulk (4-layers at a time) with every turn of the bar---which can really build up-and cause floppy sides.