I think the center line should run down the left side of the bean (upper back) and then continue to show the pinch from the buttocks that you are showing.

Just my idea, but I think it's correct because of exaggeration. If you were to "continue" the pose, you'd twist the body more and show more of the front of the body, and less of the back. I've tried showing what it'd look like without the exaggeration, and notice how difficult it is to place the shoulders (without using the robo-bean). And ultimately, the bean isn't the final product in a drawing (unless specifically practicing it). Instead, it's to build muscle and form on top of, and it's easier if you exaggerate so you know exactly what the body is doing, instead of having to guess what your lines mean.