Dose only what you can measure, that's what they say. It's true, e.g if you have high magnesium from your salt mix and your ls is not depleting them, they may accumulate to non tolerable levels.
For the first six months in my tank, ca has always been at levels of 500ppm and never drops. However with the recent addition of some sps colonies, ca have been dropping rapidly, forcing me to add ca supplement.