I pin across the seam line but I pin very little when I'm piecing. I hate pulling the pins out as I sew and I find that the pins tend to distort if I'm not careful. Generally, I'll use a pin to "match" points. Pinning is very much a matter of personal preference though. I've watched members of my quilt group work at the machine when we're doing quilts for donation and the pinning is all over the map. Whatever works for you is probably the best approach here.