Hold up, that trick works but not for the reason most people say. It's not really making the apple sweeter, @kim_martin. The salt mostly blocks your tongue from tasting the sour or bitter bits. So what you taste instead is the pure sweet flavor that was already there, just without the other stuff getting in the way. It's a cool hack but the science is a little different.
Salt doesn't BOOST sweetness like adding sugar would. It just masks the sour bits so the apple's natural sugar is all you taste. Still makes for a WAY better snack though.