Platies do not need salt to be happy. As "newtotropical" said, depending on what else you put in the tank, the salt might do more harm than good. Most plecos and other fish without scales generally prefer no salt. Neons are also sensative to salt.
Also, when you read about livebearers liking a bit of salt in their water (like molies), what is meant is marine salt, not table salt, aquarium salt, or salt tonic.
Also, even when using a "tonic salt", I have heard from many different sources that it is most effective, when the concentration of salt changes. This means that it is best to use none or very little when your fish are healthy, and add more when you have sick fish. (as the increase, or change, in concentration will help the fish). If there had been salt in the tank the whole time, the fish and bacteria/parasites would be used to it already I guess?