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.

No Quilt Police here do what is better for you

I'm a hand quilter and I sometimes stitch in the shadow (1/4" from seam).

LOL this sounds like me. It is not the easy thing folks think buyt for many years after I started quilting on the machine that is all I did and some of those quilts are still around. I have learned to meander a bit now.

There is a stitch-in-the-ditch foot for my Pfaff Creative Vision. It has a guide that you keep in the groove of the seam and it stitches right in the ditch, perfectly. And being as the Pfaff has the built in walking foot, it works great. Some other machines might have these feet, too.

One of the famous quilting experts who was on Sewing with Nancy, can't remember which one, says she uses a small zig zag stitch so that she catches both sides of the ditch. I've tried that and I like it. I've also used one of the decorative stitches on my machine, set for a wide stitch to sitd. I like that way too. I just bought a sitd foot but i haven't used it yet. Not sure how that would work since you wouldn't be using a walking foot. Anybody have experience with this foot and your backing "creeping?"

Are you kidding? If I hit the ditch only occasionally I send up a rousing shout of success! I think SID is really hard and even though I'm doing a quilt now that would really be great done in SID (if it was done right), it won't be - it will be done with meandering.

I have fallen back in the habit of stitching in the ditch, and find it much simpler than the beautiful patterns I see other people stitch.

Someone very clever needs to supply the acronym!