Larger indirect surface area on XL

I've heard about some guys laying 6 racks of ribs directly on the grates on their XL's, and I prefer to lay them on the grate vs. using a rib rack.

My question is to lay that many racks, some of the ribs would be on the edge of the grates and not have the platesetter directly below and would be exposed to the flames.

Do I just lay fire bricks in the gap areas?  Is there any reason not to close as much of that gap as possible??