This is a hotly debated subject. Many of the "old school" fishkeepers use salt.
I personally don't, as I believe if it's a freshwater fish, then salt doesn't belong in the tank, except for short-term medication.
If the fish you're thinking of is really a brackish fish, I wouldn't put it in a freshwater aquarium, as it will generally not do well and will die an early death. In general, I take a lot of the online profiles with a pinch of salt (pun intended); you need to read as many as you can, and try to sift through the poor advice. In general, looking up fish by the scientific name will give you more results from the search engine.