Originally Posted by Prism99
That is very unusual. I have never stay-stitched triangles. If they need to be absolutely accurate, I heavily spray starch the fabric before cutting. Starch stabilizes fabric so it doesn't distort during handling.
I would think stay-stitching would actually distort triangles if you are stay-stitching on the bias edge!
I certainly agree with Prism99, that stay stitching can distort as much as your final seam. Any stitching on the bias edge needs to be done very carefully. Lay one piece on top of the other without stretching .... rather than starting at one end and pinning to the other end.
I'm just finishing blocks for a quilt today with lots of squares made up of 4 half-square triangles each. To avoid that catastrophic stretch on that final bias seam, I start in the center of the block, turning the finished seams in opposite directions. Stitch from center to edge; then go back to center and stitch to the opposite edge. That way, if I do get a little stretching, at least all the triangles meet at one point and the stretching is equalized on both halves.
Good luck! :)