If you are a quilter, you know that a curved quilt edge should be bound with a bias binding.  The same applies to clothing - if you are binding the outer edges of a jacket, including a neckline, you must use a bias binding at least around the neckline.  If you wish, you can piece a bias length of binding long enough to cover the neckline into a length of straight binding as long as the rest of the jacket edges are not curved.