I think stitch in the ditch comes from hand quilting. It is easier to quilt on the side with the lesser bulk. Now with machine quilting it does not matter because the machine has enough power to go through the bulk. I do not stitch in the seam because I feel it is less strong there. I stich next to the seam, but you can do what you want, that is the beauty of making quilt, you can do it your way. There are no rules, just guidelines, feel free to bend break or follow them as you wish.