Freshwater fish do not need any salt, and as was mentioned already, some are intolerant of any salt. IMHO, don't add it unless it's necessary. Also, inverts are usually very sensitive to salt, so if you also have snails or something in there, salt would be a bad thing.
Aquarium salt can be great when the fish are sick, but freshwater fish really don't need it otherwise. I like to use it to disinfect/clean old tanks.