[tangent]
There does seem to be an urban fishy myth that livebearers are brackish water fish, though, doesn't there? I can't speak for the non-Poeciliids, but I do have experience of the Poecilia and Xiphophorus genera.
It's certainly true of mollies. Mind you, what is a molly? P. sphenops is a freshwater fish, but P. latipinna and P. velifera are brackish - especially the latter named. Problem is, most aquarium mollies are hybrids - all sailfin types must have some latipinna in them for example. So it's probably best to treat mollies as slightly brackish water fish.
Guppies? They're very adaptable, but, again, I read that their natural environment is freshwater. Nevertheless, I'm told they are often bred commercially in slightly brackish conditions. Evolution being what it is, that probably means that commercially available guppies will do better with low levels of salt than in pure freshwater.
Swordtails and platys seem to be considered as brackish purely by association with other Poeciliids. Their natural environments are purely freshwater, and a search on the web confirms the advice - no salt required.
What these fish do have in common is a preference for hard, slightly alkaline conditions - generally this means waters with high TDS - total dissolved solids. In soft water, therefore, which is low in TDS, livebearers of all types may benefit from the addition of a little salt for reasons of osmotic potential, but they'd benefit far more from a little crushed coral or shells in the filter.
[/tangent]