Technically S (stitch) I (in the) D (ditch) is just that - a stitch that is almost invisible just off the fold of the seam between two blocks or pieces in the block. In quilts being judged, the judge will look at the precision of the stitch such as the evenness and invisibiity with thread that matches the base. Stitching on the fold can be done but technically it is not in the ditch, neither is quilting 1/8" or 1/4" away from the seam. The stitch can be done on the fold but should be carefully aligned and not weave on and off the edge of the fold. A quilter can quilt any of these three methods but should remember the stitch can contribute to the overall design and if wobbly can detract. There have been some statements made indicating one can't keep the stitch either in the ditch or straight. Probably the best advice for correcting this is to slow down and keep control of the piece being quilted. I find that when I go fast then the stitch appears to have a mind of it's own and goes where I don't want it to go.