Mollies are very easy care fish for me but can be hard to are for with other people. I find that my mollies do great with straight tap water but my own tap water is fairly hard. If you have soft water from the tap, the addition of some form of minerals to your water would be a good idea. The easiest method to add minerals, one that old time fish keepers will insist is necessary, is to use some nice sea salt. The reason for sea salt is simple, your basic soft water with sodium chloride added is way too simple in terms of chemicals that are present. Sea salt, the kind that is used for a salt water tank, includes lots of different chemicals, not simply sodium chloride. That means that you are not just jacking up the salinity but are actually adding a complex mix of chemical salts that your mollies can draw from. Because common aquarium strain mollies can live quite comfortably in a pure reef tank, with the attendant salt concentrations, they will thrive in almost any concentration of sea salts. With all that said, I never add a single grain of salt to my tanks and mollies absolutely thrive in my water. My tap water is quite high in chemical content, although it tastes like a good fresh water to me. The mollies appreciate the 325 ppm of total dissolved solids in my water and go on breeding to the point that it can even become a problem to me as a fish keeper.
The basic answer to your question about adding salt is an unqualified "it all depends". This is not opinion, it is based on facts. Science never depends on opinion. Mollies require water that is relatively high in dissolved solids and if your water does not contain those solids you can overcome the lack by adding some sea salt.
