You haven't said anything about changing crank, so you realize that the type of BB you have is determined by the crank, not by what you like?

For machining, for this process, it's called facing, and it ensures that the BB ends, are positioned parallel with each other, is it worth doing? probably, but the chances are you will wear out an external BB well before you get any issues from not having the BB shell faced, however would always get this done if using a high end BB, would expect high end frames to come pre faced from the factory.