If you stitch over a seam that's pressed open, yes, it could weaken it, as you are stitching over thread only, and the needle could break the thread in places. SITD is when you stitch in or close to the ditch created by pressing seams to one side, and that can't possibly weaken a seam.

I have some quilts made by my grandma in the 80's that were SITD - some of the fabric patches have worn completely away but all of the seams (piecing and quilting) are intact and just fine. In fact the seams and quilted areas are in the best shape and it was the looser areas that wore away.

I would chalk that one up to a myth, personally.

what does SITD stand for?

SITD - Stitch In The Ditch

Sounds like one quilters opinion.