I use Rock Salt, but only when my goldfish is sick, the rest of the time it's not needed
I think salt from grocery stores are okay as long as they have no additives. Some salt's have caking agents and other things added to them, which are best avoided!
I see what you mean about the veins though.
If there are veins in the tail but no other symptoms and water quality indicates ammonia and nitrites are 0ppm and nitrates under 40ppm, then it may be that it's natural for the fish.
Usually veins in the tail are a warning sign, often caused by bad water quality and can be a sign of not only fin rot, but internal infections as well.