Stay stitching works best when it is the first thing you do. I've noticed on several of my patterns that this step is shown on the neckline after the shoulder seams are done. This is way too late. By then you have moved the garment around a lot and the neckline has already been stressed. It's best to stay stitch immediately after transferring any markings. Stitch from shoulder to center back or front on each side. I also stay stitch my knits using a walking foot. Put it near the edge so it can be trimmed off when the neckline is finished and you want the stretch back.