The border should have the same level of quilting as the rest, or it won't lie flat. One time, I made the mistake of just doing straight stitching parallel to the edges. That did not help. If I am doing plain straight quilting across the quilt, I continue it through the border. That is minimal quilting, but works. Most often, I use the border to make it's own statement, a feathered wreath, a wave pattern, a vine, or whatever. Even just a wavy line works.