Platys Mollies and other live bearers

When I was young - about a hundred years ago - I worked in a pet shop. At
that time we always used salt in the water for the Mollies, as they were
said to benfit from it, in fact wouldn't thrive without! Now I see the LFS
put salt in the tanks of all live bearers. Is this really necessary? I have
2 platys that are in great shape, and thrive in my fish soup of a tank, and
I never put salt in that tank.

Reason I'm asking is that I'd like more Platys, but are reluctant to buy
speciments that have gotten used to quite large quantites of salt in the
water in the shop. I happened to be there while they put the weekly dosage
in the tanks of the live bearers, and we're talking about a litre of salt
in 60 liters of water! Has it slowly developed to a fact that all live
bearers need salt, and not just the Mollies as time has passed, or didn't we
know any better back then?

Do I need to be concerned about their ability to adjust to life without
salt? And is it really necessary to put all that salt in the water for live
bearers other than Mollies?