Shouldn't water changes be dictated by Bioload? I have a 30gal. tank that only has 3 small fish, 1 finger leather coral, and a dozen or so snails and hermits. I change about 5gal. (~15%) once a month or so. Everything is healthy and happy and my tests come out perfect. So...I see no need to waste salt if the goodies in it are not being used up.
If there is another reason beyond replenishing calcium and other trace elements, please let me know. I don't want to hurt anything, I'm just trying to save a buck.