It is not necessary for there to always be salt in your betta's tank. Most people only add it when there is a problem present such as a torn fin or an illness. Oldfishlady also advises against constant or continual salt addition because of kidney problems it can cause in your betta, I believe.
If you are adding it, though, you should only add it with water changes, and not when replacing evaporated water. Because even though your water is evaporating, the salt is not, therefore leaving you with the original concentration. Or something. Terrible explanation, I know.