I think, the biggest key to additives is to look beyond the advertising. Allot of stuff out there tells you that you need to add it, but doesn't offer any form of information with regards to what it does in detail, nor how to test for it. Much as the guys have already said, generally I never dose anything other than what i can test for and only once i have found a definite need for it based on research to rule out any advertising jargon.
I always look at it this way...(lighting, filtration and food aside) its been proven time and time again that it is quite possible to have a stunning system with thriving corals on nothing more than good main core elemental supplementation (Alk,Mg,Ca) via whatever method you choose to supplement them, plain old regular water changes, and a sensible stocking regime. Anything else over and above this is in most cases purely speculative, and quite often the results of allot of additives are 'to put it mildly' open to extreme cases of placebo effect on behalf of the user...ie is that coral 'actually' growing better or is it assumed to be growing better because its now swimming in snake oil..:scratchch
regards
Simon.