There are people who do it both ways. Some trim the fat to 1/4 inch and leave it on top. The other school does not trim anything and puts it on the bottom. My charcoal grill sucks and I normally use it as a smoker, so my thought was to put it on the bottom. From reading, the fat juices are not able to penitrate the meat fibers, so on the bottom it gives a barrier to the uneven heat I am applying.
I could be 100 percent wrong, but? Its my first one. If its bad you guys will know