Adding salt is a personal preference IMHO. There are good and bad about it, depending on your needs. Hope these links can clear your doubts...
http://www.drtimsaquatics.com/should...water-aquarium
http://www.aquaworldaquarium.com/Art...r_Aquarium.htm
Personally....20+ years ago I do add abit salt to my arowana tank. Nowadays, I never add any (except treatments) to any of my tanks.