Before you handle a jacket or shirt front and back pieces too much, you should staystitch the neckline to prevent stretching.  Staystitching is simply sewing about 1/8" inside the seamline with a relatively short stitch along any curved or bias seam.  You can clip the seam allowance of a curve at this time also, at 1/2" - 3/4" intervals, to near the staystitching.