Your usage of the terms "Up and down" vs. "straight across" confuse me.
Do you mean: When pinning units together for piecing do you pin perpendicular to your seam line or parallel to it.
If so...I pin perpendicular to my seam line. I can't imagine pinning parallel, unless you are hand piecing and even then that would be a bit cumbersome because you would have to remove the pin long before you get to that area for stitching. By pinning perpendicular to the seam line, especially when machine piecing I can stitch right up to my pin before removing it. In some cases I will stitch right over it if the placement is such that I am sure my needle won't hit the pin.