Stephen King claims to rarely know how his stories will end. Misery sticks out in my mind: he mentioned it in "On Writing" and said that he had no idea how the story would end, until writing the last couple of chapters. While I'm not a huge fan of his prose... he is a commercially successful author that knows how to appeal to the masses. His biggest advice is to start with an idea, or scene, and see where the story takes you.
I think it's good to know where you're going and therefore be able to communicate to your audience where you're taking them.
