In my experience it all depends on your heat source. If your heat comes from below then fat down. If the heat comes from the side and passes over the top of the meat, fat up. This will help protect your meat. My cookers heat from the bottom, which I like best because that means I will have fat down and form a really nice bark with no grill marks.
FEC120, XL BGE, UDS, Cimarron, Weber Jumbo Joe, Weber Performer Platinum (Copper)