I was reading an interesting article on fancy goldfish farms and breeding and how after they had realized salt would kill many of the diseases like flukes and such, they started to salt all the time. Over a period of time the farms began to notice they were getting breakouts of salt resistant 'bugs'. Where at ont time they could treat the water effectively and cheaply using salt, now they had to spend more from other medications that are harder on the fish as well.
I still believe using salt for certain fish is good, even needed, but if you are trying to do this to prevent disease I don't know if I would anymore. I favor high water changes anyhow, and if I treat with salt at some point I just do water changes with plain water until I need to salt for something again.