What kind of soup are you making? For a bean or lentil soup, I salt early so they don't stay tough. For a simple broth with veggies, I add a little at the start but adjust right before serving.
Honestly I go the other way from amy_henderson83 on bean soups. Salting at the start can actually toughen the skins and make them take forever to cook through. For a simple tomato soup, I add zero salt while it simmers because the tomatoes reduce so much. I wait until the very end to season. That way I don't end up with a salty mess if the broth cooks down.
My grandma's bean soup once turned into a science experiment because I salted early and the beans stayed like little rocks. After three hours I admitted defeat and ordered pizza. Now I wait until the beans are fully soft before adding any salt, then I season at the very end. I'm still not sure if the early salt actually toughens them or if I just have bad luck with beans. Probably the latter, since I also managed to burn water once.