I would say that there is some evidence that salt can help with ich. The idea is that the presence of salt stresses everything in the tank, the fish and the ich. But, the ich, being a much lower life form than the fish, succumb to the stress earlier and more fatally. That is, the salt only stresses the fish and doesn't kill them, but does succeed in killing the ich. That said, all the caveats I listed above and in those previous threads I referenced still remain. There are species that are very intolerant of salt -- it doesn't do much good to kill the ich by injuring your fish. Fish whose natural habitats are very low mineral are going to be exceptionally intolerant of salt -- the cure in this case may be worse than the disease.
Not only that, but there are strains of ich that do not respond to the salt treatment at all -- that is, some strains of ich survive just fine all the way up through brackish water. My usual recommendation to people who want to try to use salt (almost always coupled with the heat treatment, too) is to try it for a few days, but if there isn't significant improvement you need to have medications ready to use. You can't know before hand how the ich will respond to the salt, so you have to have medications ready in case the salt is ineffective.
So, salt can help, sometimes, with ich and other parasites. However, it is not a magic bullet. And, for a lot of parasites, there are medications that are faster, more effective at killing the parasites and safer for the fish, to boot. So, yes it can help, but it may not be best choice. Again, research is needed to know what medications are available for the diseases.
