The use of salt is really subjective and most of it doesn't really stand up to scrutiny.
Example, I could start putting a spoon full of koolaid in my tanks every water change. The fish seem to like it, they seem active, they seem happy, therefore koolaid must be healthy for them.
Much of the reasoning behind salt use in tanks comes from a misunderstanding of the term 'salts'.
People say to add salt with mollies because some mollies come from estuarine conditions (i.e. brackish). Mollies are perfectly adaptable to fresh or salt conditions, though, and adding a dash of sodium chloride does not make brackish water anyhow.
So if you want your mollies to go back to brackish estuarine conditions, add marine salt mix, but make sure their tankmates are brackish tolerant also.