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If the heat source is offset, then I'll do it fat-side up, so it's "basting" the meat. On my WSM, I place it fat-side down to shield the meat for the more direct heat.
I do all my meat with the fat up as I like the juices to seep through it;however, take a look at Al's thread on the ham he did. I will try that sometime so I may save time at the table
For me it depends on which unit I'm using. If it's the Lang then it fat side up. If it's the GOSM, the ECB or the electric UDS it's fat side down to act as a buffer on the occaional heat spike-maybe not so much with the electric but then habits are hard to break.
I have always cooked butts on my WSM with the fat side up, I am with the train of thought that as the fat cap begins to melt it will help keep the meat juicier. Plus I feel like it can't hurt to have the exposed side of the shoulder bombarded with rising smoke!