I wouldn't say that's true. Aquarium salt is pretty useless stuff (although it can help in treating ich). I have heard that mollies do better in mildly brackish water than in pure freshwater, but for a brackish tank you'd want to use marine mix, not aquarium salt.
The key to keeping healthy fish is clean water. So long as you buy healthy fish, put them in a fully cycled tank and keep their water in tip-top shape via regular water changes, you should avoid most problems. There are always "diseases" present in the tank but keeping your fish happy and stress-free keeps them from coming down with illnesses (most of the time, anyway).