I am not aware that this is so, but it might apply to some spirals. It would not apply to most of them because spiral arms are not the smooth pinwheel patterns than many people imagine them to be, and it is easy to 'loose' an arm and end up on an arm 'spur' or fragment when you are looking for a mathematical pattern. Like Bode's Law for the planets, they suggest something special about the asteroid belt when they are supposed to only apply to real planets.