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I'm no expert by any means, but here's my thought: it could be done just about anytime. You could think about it before you start writing so that you don't have to go back and change your tone, or you could get all your thoughts out so you don't forget them and then edit them later, or maybe you could just write until you get stuck and then maybe as you're editing what you have so far you'll get more ideas. I think different people have different styles and some methods work better for certain people than others do.