normally they are filed/ground beforehand, because in the case of simple designs like flat plate you can get at them easier and burn off some metal more readily while they are separated, you don't risk scratching your tubes. And in the case of fancy stuff there are sometimes little features you need to get at with a file, that you can't do once brazed. Not to say it can't be done either way, or you might not leave a little bit extra to blend in.