Real salt (sea salt with trace minerals) is a healthy and necessary part of the human diet.
The problems are that 1) most salt people eat isn't healthy salt; and 2) processed and prepared foods and restaurants foods radically over-salt things.
Our advice is to never eat processed prepared foods, and eat at restaurants as little as possible. At home, salt things conservatively to taste, and you'll get about the right amount of salt.
Endurance runners and other athletes need more, but that's a special case that we devote some attention to in our upcoming book.See here for a related post by The Spartan Diet on Processed Foods: The good, bad, and ugly.