My understanding is that the 'ditch' is created by pressing to one side; you then have one side higher than the other, and the lower area becomes the ditch. You then stitch very close to the stitches on the ditch side.
If you take the above view, you have no ditch to stitch in if you press your seams open. If you stitched in the middle of a pressed-open seam, you would be stitching on thread, not on fabric. I don't see how that would do anything for your quilt. So if you do 'sitd' on open seams, I would suggest going slightly to one side or the other of the seam line.