i found that after my platies got some salt in their water they were a lot more livelier (sp?) so i recommend it, i didn't even put the full dosage that the salt box said to, just half and they seem to love it.
It is up to you I guess - I've heard some people adding salt into livebearer's tank before, but personally, I don't add any and I don't have any issues with platies. As you can see from my sig, I do breed a small number of them so I have over 50 at any given moment and so far, I didn't have to add salt...
yhbae, you are correct. The true platy is not 'brackish' in a technical sense. However, if you look up on www.fishprofiles.com it lists them as benefiting from salt in the water.
But in all probability the Platy that is commonly sold is more of a cross between a Molly and Swordtail, which are brackish water fish.
Just went through goodle.com - cound't keep my curiosity away for long...
From what I have read about platies (I did it over a year ago, don't remember anymore), most of the articles/profiles don't mention anything about salt but some of them do mention that it does benefit to have some salt in the tank. Of course, this is assuming that you don't have other species that don't like salt such as catfishes...
So for me, I guess I could add some in my platy-only tank, but so far, I haven't lost any so why mess with something that works?
But I do hear others talk about adding salt to their community aquariums, whether they are livebearers or not.