There are no regulations as to how much salt is in salted butter - it could have hardly any, but it can have as much as 2% salt in it, which works out to quite a bit. If you are trying to control your sodium intake, it's better to not use salted butter and add as much salt as needed.. . Also, salt is a preservative, and butter with salt has a longer shelf life. Therefore, it's likely that the unsalted butter on the shelf in the grocery store is fresher than the salted counterpart right next to it.