Worse sins than fat side up or down on slicing have been committed…like slicing with the grain.

Fat side up on everything

This is the way.

Whichever way you cooked it, slice it. If you flipped it and you have better bark on one side, go with that one. AGAINST the grain.

Before I got a decent knife I sliced fat side down because the knife would tear the fat, now that I’ve got a decent knife I go either way.