when baking and the recipe calls for butter, do use salted or unsalted?

I always use unsalted, as the salt content can vary from brand to brand. Most recipes I bake with call for unsalted, so unless the recipe calls for salted butter, I would use unsalted.

Either if the recipe calls for salt. Just use less salt if you're using salted butter. Otherwise, use unsalted and adjust salt later.

I would recommend using unsalted butter for all of your cooking, not just your baking. It will allow you to control your seasoning much more accurately.

Almost always unsalted. An occasional recipe specifies salted.