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Fat side up will self bast the butt in it's own fat drippings to help retain natural moisture.
Fat side down will help protect the meat during temp spikes if you have any problems with that during the smoke. I think alot of folks use fat down with charcoal fired drums and bullets, just in case.
I have tried up, down, and trimming the fat cap off. They all came out the same. I use a water smoker, so everything stays nice and moist anyway. So I trim the fat now.
If you have an off-set or drum, maybe it would make a difference (??)