Hygiene might be a bit much, I think if you'd just do assisting you'd get the same benefit. I think either would go a long way to showing how dedicated you are to dentistry (I did a assisting class and internship in high school).
It won't necessarily make up for bad grades in college or a low DAT score, but it will definitely give you a point or two over people who don't have significant dental experience. This can also be a way to knock out most if not all of your shadowing hours - as well as give you the real insight into how a dental office runs and if you want to make it your profession for the rest of your life.
Good luck!