I agree not to use Bettafix or Melafix (which is just a more concentrated form of Bettafix). I've read that it can damage the labyrinth organ, and though everyone doesn't agree on this, I always err on the side of caution.
Personally, I don't even use salt when a fish is healing a wound. Unless there are signs of infection, I just keep the water crystal clean and absolutely free of ammonia/nitrites/nitrates (as always) and feed them a good quality diet (I only feed frozen foods, a variety of 6 different ones). This has been the best recipe for healing in my fish.
As bettas are freshwater fish, I don't use salt unless treating ich. (which I've never had to do). From what I've read, salt will function as a mild antiseptic at best, and at worst can cause osmotic stress in fish that come from 0 salt environments (like bettas) and overuse of salt in home aquariums has already produced salt-resistant strains of ich.