I'm often asked about butter - what type to use, how to soften it quickly, should it be salted or unsalted - all great questions. And because I know a lot of people wonder the same - the most common is, should I buy unsalted butter for a recipe that calls for it? I thought a more detailed answer was in order.
In short, unsalted butter is often called for in recipes so that the cook can have complete control over the salt in a recipe. But in reality you'd rarely make anything - particularly baked goods - without adding at least a pinch of salt. So I don't see much point in buying unsalted butter - which is more expensive and doesn't keep as well (salt is a preservative) when you're going to add salt anyway. (My two cents.)
On the other hand, I do love unsalted - also known as sweet - butter. It's creamy and sweet and fresh tasting spread on bread (or crackers), straight-up or with a sprinkle of coarse salt on top. But the fact is, regular salted butter is far more common, and so I default to it. (Ditto all-purpose flour - I generally don't do recipes that won't work without cake and pastry or bread flour, because most people just don't have it in their cupboards.) The average pound of salted butter contains a teaspoon and a half of salt, just to give you an idea - and most recipes call for somewhere in the vicinity of a teaspoon of salt, but I rarely use as much. You do need some, though - salt enhances the flavours of the other ingredients, ties them all together and keeps the whole thing from tasting flat. If you ate a slice of bread in which they forgot the salt, it would be noticeable - it's not that regular bread tastes salty, it's that if it didn't have any salt, it would taste as if they left something out.
Some argue that they can tell the difference when baking with different varieties of butter, which is true; you'll more likely notice when making more delicate or finicky things like pastry or shortbread, and it most likely has more to do with a difference in butterfat content. The butter we have access to in Canada, made simply of cream and salt (or not), must by law be 80% butterfat; in comparison, high-end and European butters range from 84%-88% butterfat. You'll sometimes see "creamery" butter on store shelves, or "European-style", and sometimes they actually list the butterfat content. Unfortunately, fancy butters are often twice the cost of the basic store brand - I'd suggest the cheaper stuff for baking, and the pricey stuff when it's going directly into your mouth, tossed with pasta or spread onto a great loaf of bread.
No matter what kind of butter you choose, if it's cold, you won't be able to cream it - to soften it quickly and evenly, grate it with the coarse side of a box grated and let it stand for a few minutes, and it will be perfect to work with. (Similarly, you can grate cold or frozen butter directly into pastry or biscuit dough, rather than cut it in.)