Should I put some salt in my freshwater tank?

In my 20 gal tank I have 3 mollies, 4 platys, a pleco, 3 neon tetra and a few live plants. I've heard that a little bit of salt in a freshwater tank is a good thing (especially for mollies). Is this true? Should i put some salt in my tank? And if so, what are the benefits to doing this? Thanks!