The only way salt is lost from the system is in skimmate, salt-creep, and when bagging frags. So generally all you need to add is pure water with top-off (only pure water evaporates). It's good to occasionally remove salt water and replace with fresh mixed salt water to remove accumulated junk that you don't want in your tank, and more importantly to replace depleted trace elements that get used up in your tank (things like iodine, strontium, molibdinum, etc, that we don't test for but may be important for our creatures (did I make up that last one?).
Also, I find that my salinity drops slightly between water changes because of the skimate I remove, some salt-creep, and saltwater removed to bag frags. So when I do a water change, I mix my new water slightly higher than I want the tank, and that tends to keep the tank salinity from dropping over time.
Nate