One of the reasons to stitch in the ditch is to create texture , but not visable stitches. So I go right into the seam with invisable thread. Echo quilting is right next to the shape you want to give texture , and it truley outlines the shape you want to give empahsis, and is typically about 1/8 to 1/4 inch from the edge ,and continues out in rows. Echo quilting is a great way to draw attention to the quilting. A stitch in the ditch is give texture without visually distubing/interupting the quilt top design. Some may use a decorative , this utilizes the seam as the quideline for the stitching.
SOme believe that stiching in the ditch , right in the seam, may cause a break in the seam thread. I have done many many quilts stitiching in the seam and have never had any issues.