If it is 1/2” or less, leave it on as it’ll supply moisture and render itself away during the cook. If it is thicker than that, trim it off in one big hunk, add the rub, and then replace the fat cap, and cook fat cap up.
However, the fat cap down on a WSM is a unique idea. I used to have a smoker I built out of terra-cotta flower pots and the bottom of the pork shoulder always got turned to charcoal from being too close to the heat source. I’d probably just use some foil and stil place up.
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