I would recommend buying salted butter when the recipe calls for it. Adding salt to the recipe doesn’t incorporate it into the recipe in the same way that using salted butter does.
Also most recipes I’ve come across that call for salted butter are calling for European style butter. American and European style butter are not interchangeable as American butter has a higher water content.
I keep American style unsalted butter and European style salted butter (preferably cultured) in the freezer at all times.
