Actually, salt was added to butter originally in the days when there wasn't so much refrigeration. What sold in the stores now as salted and unsalted butter is identical except for the salt content. It is true that if you keep your butter in the fridge and use it mainly for baking, it is better to get unsalted. If you're going to keep it in a butter bell, you need salted.
What sold in the stores ? Not around here, I buy my butter from a farm!
You don't need to store butter in a bell, salted or otherwise... you want a butter crock for that, as the sunlight affects the shelf life too :) http://www.webexhibits.org/butter/off.html
