I usually press to the dark. It tends to hide the seam. If I am hand quilting, it easily "marks" a stitch line for me and it helps "puff" the block a bit. If I'm machine quilting, I prefer to not have the seam pressed open. If the needle hits the stitched line--which is the intent with stitch in the ditch--it can break that seam stitch, as Tartan mentioned.
As for pressing open weakening the seam--remember that we used to press the pants seam open! Of course, we sometimes double stitch from one notch to the other to strengthen that curve, too.