I prefer a printed backing--it hides your mistakes. I would have the seams vertical depending on the fabric. If the seam makes too much fabric in one place then iron the seams open. I have done it both ways and don't notice much difference.
At this time I have been looking for a very light (not white) backing for my OHMIGOSH quilt. The background is white and cream and I'm afraid that any pattern will show through. I do have plenty of time since it is third or forth on my list of quilts to be finished.
Sue