From what I've read, using a SID foot with the metal guide running precisely along the stitching line puts the stitching just to the right of the seamline, which is correct. According to SID info., if you stitch exactly on your stitches, it weakens the seamline stitching; therefore the stitches SHOULD be just to the right of the seam. My SID foot works that way. (IMO, without a SID foot/metal guide, it would be virtually impossible to stitch a straight line next to the seamline.) If you have the SID foot and still sew crookedly, one thing you need to be careful about is sewing over "lumps" where two seams meet. Those lumps can cause your guide and stitching to go wonky. Also, when using the SID foot, the narrowest zigzag stitch works well.