Low to no salt is best, they are one of the fish I do not keep with even a little salt, treat them like neons or cardinals, soft, saltfree, slightly acid water is best.
Salt is good for injuries. If you have a shoal of small pygos (or larger ones for that mater) there is a chance one of your fish could end up with a large flesh wound from one of its tank mates. Though the healing power of piranhas absolutly blows my mind, adding salt temporarily to the main tank or putting the fish in a quaruntine tank with salt helps healing and to fight infection.