If only people were rational enough in their behaviors and efficient enough in their communication for that to really be the silver bullet.

In a small company, there are rarely such neatly demarcated lines. Regardless of how well you communicate with your boss about arranging to be done earlier, you will feel the eyes of your coworkers when you head out the door.

edit: A notable exception being results-focused cultures that emphasize flexibility as a company value. If punching the clock is an expectation, it's hard to manage to be on a shifted schedule without there being at least a twinge of resentment when you head out earlier.