The amount of salt in salted butter can vary enormously and may be as much as 3%. In general it is better to cook with unsalted butter so that you have more control over how much salt is going in to your eventual dish. However, the reason for salting butter originally was to preserve it, so unsalted butter does not keep as well. However, in this day and age of good refrigeration and large consumption, this is not the issue it was.