Good topic for discussion! I will stitch on the seam or nearly so if my intention is to make the stitching disappear. Lately that has seemed less important, so I try to keep a constant distance from the seam, and will often stitch on both high and low sides to keep that distance when stitching inside or outside of a design element. I've found a zipper foot can work nicely for this, providing better visibility, and haven't had any wrinkle problems as long as the quilt is well pinned or basted.