When you are working with a stretchy curve on a garment, especially a neckline, it is very important to stay-stitch the seam allowance as soon as possible when you begin handling the pieces.  Set your stitch length to a shorter stitch than you are using for construction - quilters would recognize a 2.0 length, and stitch 1/8" inside the seam line (1/2" from the raw edge) on all inside neckline curves.  Then carefully clip the seam allowance every 1/2" - 3/4" up to, but not through, the stay-stitching.