When I do water changes, I often take out the filter cartridge and dump the salt into the filter then turn the filter on it. The filter pumps water over the salt, dissolving it with little work.
You can apparently use this method when you add salt with fish in the tank too, because it dissolves the salt at a rate that normally does not bother the fish and distributes it around the tank.
Also, I think it depends on how small the salt crystals are. If the salt crystals are small, I think there's the possibility of a fish breathing it in and it getting on their gills and it burning them. Larger salt crystals probably might not have that problem, although my concern is if they try and eat it and they choke on it or something since it's solid and not soft :S