In the instructions for creating the applied collars from the neckline measurements, the neckline edge is drawn as a straight line from the CB to the shoulder point. When this straight line is on the grainline (as in the image below), there is no flexibility given the nature of the straight grain.
You can finesse the neckline a little to give a bit more flexibility and shape to the neckline by raising the neckline point by 3/16-inch (0.5cm). This means that the neckline is no longer on the grain and has a bit more flexibility. See the image below with the Basic Collar and the Turnover Collar as examples. The same principle will apply to any collar.