I"ve been stitching in the ditch and press open, never had an issue with it weakening the seam. I think its easier to get really in the ditch with an open seam. As far as batting migrating, this was an issue before batting had a scrim or well bonded (poly) . If you are using a batting with a scrim it should not be an issue. Some bonded poly is not well bonded and it will migrate and beard like an unbonded poly.