Most people don't salt at all unless they are treating a sick fish.
However, I do salt my tanks. I have some fish that are near brackish (mollies), and have difficult tap water, so do partial changes with RO/DI and feel the salt helps raise the electrolytes, in addition to adding trace element mixtures (you can buy trace element compounds for RO/DI).
I also feel it is good prevention. There are many schools of thought on salting tanks.
I dose a palmful per 10 gallons, directly to the tank rather than dissolving it prior to addition, as this allows the salt to dissolve gradually, rather than all at once, as the salt dissolves in the tank.
on partial changes, i adjust accordingly, for example, if changing 50% of a 20 gal, i add 1 palmful of salt.