I like to put the fat cap down in both my offset and vertical for the one reason that I like to have a nice unblemished surface of bark. That way the grate doesn't ruin the only side where the rub matters the non fat cap side. I have tried it both ways and they always come out great fat cap down.
Assassin gravity feed smoker
Custom Pitmaker Magnum Sniper Smoker
Weber performance Kettle