Always cook with unsalted butter.. I am not a baker and there may be reasons to use salted butter when baking but never when cooking. I have sold food to commercial food operations for over 20 years and I have never, not once sold a case of salted butter. In food distribution butter is sold several ways but the most common is a 36- 1 pound case of butter solids. On average my accounts purchase 20 cases a week from me. Over 20 years that is more that 14,000 pounds of butter I have sold and not one single pound was salted.
I recently asked one of my best Chefs about this and he explained you can control the salt content of all dishes directly by adding salt to recipe and salted butter varies in salt content. I suspected this was the reason and I’m glad to confirm it.
He did say if you were going to create a compound butter or want a salted butter to spread on bread you could, he supposed use salted butter. However he added why would you when you can add salt to your desired level.