In sewing classes, sewing reference books etc we've all been told to stay-stitch these areas to secure them from stretching, works well some of the time. Here is a fabulous rag-trade tip that works much better for this problem.
Tear-Away Vilene is a non-fusible (soft paper like) interfacing, available in Black and White. In the fashion industry patterns usually come with vilene shield instructions for necklines, armholes and waistbands to be cut as well.
Instructions: After cutting your pattern out in your chosen fabric, you lay the neckline, armholes and waistbands onto the vilene, pinning it in place and cutting around your outer neck, waistline or armhole (only if it's sleeveless) edges and approx out 1" past this point, then you mark approx 1" down from this, these are your vilene shields. You end up with something like this.
In a dress I made recently in this very lovely, but soft and floppy rayon, having the vilene shields attached to the neckline, made it so much easier to handle when sewing on my neck bias binding.
Pin in place and sew it (within the seam allowance) onto the wrong side of the neckline front and back, you can sew the shoulder seams together and after sewing on your neck facing/binding and just before you turn this over to the correct side you rip into the tear away to the seam line and tear it all away, what ever remains inside the seam can stay there and will be covered up by the binding or facing. It will also tear-away from the shoulder seams.
Vilene shields were also added around the waistline of some linen pants I've been making.
After attaching the waistband facings and before turning the facings over, I tore off the vilene shields.
Simple and easy as that, you sew the vilene shields onto the wrong side the edges you want secured, after attaching your neck/armhole/waistband facings or bindings, the vilene then gets torn off. You will find you will have a neater edge with no stretching occurring around these edges, far more effective than stay stitching, don't you think?!!
NOTE: Ensure that you get the correct weight soft vilene, some thicker weights do not tear-away easily.