WEll it would depend on WHY you're adding it. For livebearers such as mollies/platies/swortails, they appreciate some salt at all times.
For salt to be effective as a first defence against suspected disease, the tank really shouldn't have salt in it all the time.
Some fish like loaches and plecs, silver-coloured fish, and scaleless catfish are SENSITIVE to salt..they can tolerate it.
I'd keep some handy at all times, but the fish in the tank will determine the use of salt regularly or as a treatment.