A bit more about shaving the neck that I just though about, (recognizing that my neck might differ from yours)...

My instinct was to look UP in order to stretch the skin tight under my neck. And for most of my neck, especially directly under my chin and extending partway to the tip of the jaw under the ear, this is the proper technique. But, stretching my jaw and face upward was the wrong approach for the area of my jaw where it angled sharply upward, directly under my ear. I am skinny and angular, and when I stretch my face upward, the angle of the jaw is accentuated into a 90 degree angle. It was very easy to nick and cut while shaving around my jaw then! And, worse, it was at what is probably your blindest spot.

The cure for the upper portion of my jaw was to look DOWN while shaving. This filled in the gap between jawline and neck with flesh, giving me a clean angle to shave across.

A small warning though...

If this is your case too, and you look down, note that the filling in of the angle between jawline and neck might also result in skin that is not entirely flat, and is also easy to cut. You might have to look down, AND stretch your skin at the same time.

Experiment...