Stitch in the ditch in one direction only would work, as you have no batting to worry about. If you just want to, it looks like it would be easy to do in both directions as it lines up both ways. You also might consider a curvy line in the white row. Often, that's hard to do consistently unless you mark your quilt, though. If you match the thread to the white, not being consistent wouldn't show very much.
bkay