I'm another fan of a wide serpentine stitch if you have it as a built-in option. It goes well with everything and you can set it/choose threads so that it either disappears or becomes part of the grid/project. I think using the seams as a guideline (no marking!) and going over into both seam allowances stabilizes the quilt and holds up well to being machine washed.
I know -- some of you can do it free hand, that just makes me cranky! Not that you can, but that I can't. I can, however, let my modern machine do most of the work -- didn't have that option with my vintage.