The problem with mercury Amalgams "starts" when the mercury begins to leach out of the filling and into the blood stream; this happens slowly but surely as your filling starts to degrade (your saliva is enough to do this).
Many people advocate not to start chelation until your mercury fillings are out because chelating agents will "hunt" for mercury and not only will chelate mercury in your blood stream but will also chelate the one in your fillings.
The problem with this is that chelation can pull out mercury of the Amalgam too fast and actually increase your mercury levels, causing you to be sicker; again, mercury that is bound in the Amalgam is "safe" until begins to leak into your system. Chelating agents can overdose your system with mercury if there are large sources of it. Obviously if your elimination organs are in prime condition you might not notice the effects of chelation with Amalgams but its certainly not advisable to do so.
Mike