I almost always press the seams to one side.
Well - that was interesting - I just "looked closely" at some of "ditch" quilting from several years ago. Most of my stitching is not visible from three feet away. However, when looking at it from about 10 inches away, I was wandering all over the place.
I consider "ditch stitching" to be as close to or on the seam line as possible.