I prefer it the way it is. How hard is it to hit the zero key to un-assign? Keys that toggle an assignment on or off would be more prone to input error. For example, while making color class assignments in slide show, I enter "1", or did I only half press the key. Now I can't tell if I am toggling on or off without visual reference from the screen. Now I need to move forward one file and then back to check the color class. My opinion only.