It's a personal choice, of course, and it sounds like you are leaning away from it. Keep in mind however, that one of the main purposes of NB is to rid the body of toxic metals. It will be very hard to do that while keeping your toxic fillings in your body. Check out these articles: http://www.arltma.com/Mineral_Information/Mercury.html
It seems reasonable to conclude that if you are increasing your body's ability to excrete toxic metals that you might make it a lot harder on yourself (and possibly quite sicker) if you let the fillings remain since your body will be encourage to draw out those toxins.
Your choice, of course, but something to think about. Best to you!