Aquarium salt is good to have on hand. Actually both that and epsom salt should be on hand, they don't expire so you'll likely have it for the lifespan of the bettas. But it's better to be proactive than reactive!! You don't need to use the salt all the time- only if they get sick or bloated. I have not seen either of my veiltails ever get bloat (cross my fingers) but my crowntail mix gets it all the time and I have to be careful with him.
:) Learning everything about bettas is fun ^.^