"in" the ditch means in the seam. if your seams are pressed to one side then you have no choice but to sew to the other side of the press.
if you have pressed your seams open, then you stitch over the stitches so the quilted SID line disappears into the piecing line. this is a bit chancey unless you pieced with really small stitches and a very sturdy thread. otherwise, if the needle pierces the thread in the piecing seams, it could weaken the structure of the whole quilt. (unless you have a super-eagle eye and can make sure the needle goes always between the piecing stitches. sounds like a good way to torture yourself. :lol:
on the other hand ... man! would you have braggin' rights if you can pull it off!
good luck with that. :lol:
i have switched to "as close to the ditch as i can get". :lol: