This question came up today in reference to this week's recipe of the week, apple pie. The recipe calls for unsalted butter. Someone wanted to know why that was. The truth was, I wasn't sure. I always use salted butter for baking because that is what I always have on hand. Now curious, I set out to do some online research. Every site I came across states to ALWAYS use unsalted butter in baking unless otherwise stated. A couple reasons given....first you have more control over the salt in your recipe. Also, the unsalted butter is better for the texture of your baked good.
As I stated, I always have used salted butter and think my baked goods turn out yummy, never too salty. That being said, I think I will now start baking with unsalted butter. Apparently it's the thing to do.;)