Epsom salt tastes horrible. I don't use it anymore unless building my own water from distilled, and even then, only use a fairy-dust sprinkle amount (like 1/10 teaspoon) and no greater.
I've never added salts late, but as a chemical engineer and beer nerd in general, my advice would be... use less than you think you should. Like, calculate how much you think you should use, and then only add like 1/3 as much at a time until you achieve a perceptible difference in flavor, and then stop. Same goes for additions of all different kinds -- spices, fruit extracts, bourbon, you name it. Go easy on the special additions.