'Salt' is a very broad term. In freshwater most people refer to 'aquarium salt' which is not much more than sodium chloride which will not alter pH/KH. Marine salt, which some people will use in freshwater does alter pH/KH.
Most tap water is pretty hard/alkaline so it doesn't need any help from any kind of salt. The only exception IMO is with rift lake cichlids because their native waters are so hard IME they do better when you buffer the water and add special cichlid lake salts.
Brackish obviously needs a lot of salt, but it needs to be marine salt. There is some debate about whether mollies need or benefit long term with salt. They are freshwater fish who in nature will go in to brackish and even marine water in search of food. However, this is for short periods of time, not the only option for their whole like (like in our aquariums). The evidence is pretty anecdotal about salt's benefits with mollies. Since they are usually in with freshwater fish they should not receive salt all the time. Long term exposure to constant salt causes damage to the kidneys and other internal organs since we are forcing a salinity on them that they are not adapted to in nature.
I definitely recommend keeping aquarium salt on hand, but only using it as needed to treat illness.
