I stitch right in the seamline on wall hangings or doll quilts if I want to hide the stitches.
If I want to strengthen the seams, I use a very narrow zig zag stitch and catch both sides of the seams. Nancy Zieman calls this a wobble stitch :wink:
Hand quilters usually only stitch just to the side without the seam, to avoid the extra thickness of the seams.
If your machine can handle it, you can stitch on the bulkier side of the seam line.
Stitching truly in the center of the seam, does not help to strengthen the quilt, as you are stitching on just the thread from the seams, not the fabric itself :D:D:D