Like Rbursek noted quality of salt is important, we naturally assume WCs are primarily for nutrient/detritus export but just as important is Ca, Mag, Alk buffering, and trace elements addition. So, if you are dosing for Ca and Mag doing WCs can actually hurt if the Ca and Mag elements of that salt are lower than what your tank parameters are. You spend a few days getting your numbers higher only to lower them with a Water Change that has less of what you want.
Along those lines if you're not using RO water then WCs are just adding dissolved solids, phosphates and whatnot - so again you may be using a Phosban reactor or some sort of power filter which gets your phosphates lower but now a WC with water straight out of your tap adds it all back in.
One of the things I'm trying to figure out is the addition of top off water, I'm dumping 2-3 gallons a day of basically inert RO/DI water into my tank, in effect I am doing a 3 gallon water change everyday with distilled water. What can I put in that water to make it a "positive" addition to my tank possibly cutting down on my frequency of WCs....options include limewater, a buffer of some sort, maybe saltwater every third week or something, how 'bout Ca and Mag, I'm sure there are more.
So my long answer to your short question is there are many factors which you should consider when trying to figure out how often to do WCs....stability of the system may be most important of all rather than chasing numbers. How about costs.....you can save some money by buying a salt that has what you want in it thereby cutting back on the need to dose or how much product to dose, potentially cutting back on WCs your system can be more stable, having corals that thrive in a little dirty water, having less fish to make it dirty, (which requires less food = less cost) or adding critters that help keep things clean, and then you got the whole how much time do you have to spend on maintenance, etc....
Sorry to ramble, one thing I have learned that even the simplest of questions has alot of different things to consider. That's why I enjoy R2R so much....
