Im not certain if there is a best method. I read through the instructions and didn't find anything when I first got mine (though I sort of just skimmed them quickly)
As for what I do: depends on timing. if i have no reason to need the smoker cooled down, ill just dump out any extra chips/ coals (if id used the chip tray) and shut the door, open the chimney and let it do its thing. On the other hand, most of the time I need to put it right away which involves sticking my cover on it and putting it in a small area in the garage between potentially flammable things. So usually I will just leave the door open so it cools faster so I can put it right away.
I cant imagine that this would cause big problems, but as there could be some expansion and contraction issues, perhaps closed is the safer bet. Anyone else have thoughts, or know the "proper way"