The food-snob answer is that you should generally be buying and using unsalted butter, because with it you are better able to control the amount of salt in your cooking and eating. The quantity of salt in salted butter also varies somewhat from brand to brand, so it can be a little challenging to know exactly how much salt you should leave out of a recipe.
the same time, there is such a distinct taste difference between salted and unsalted butter that you might not be willing to switch to unsalted butter (an otherwise delightful child we know has a fit if salted butter is not available for his toast, and unsalted butter sprinkled with salt only increases his disdain).
If you assume your salted butter has three-quarters of a teaspoon to a whole teaspoon of salt per stick (one-half cup), you'll be erring on the side of caution in decreasing the quantity of salt you add to your recipe. You'll want to taste whatever you're making along the way to determine whether you have enough salt for adequate flavoring.