porcelain briquet's or lava rock or nothing?

Kaylora is correct for most BBQ's. You say "at the bottom" and not over the flame bar, I've seen these types of BBQ's that have defusers over the heating tubes and rock underneath. Those types are to catch any drippings and keep them from pooling which when heated can flame up and cause a fire from the bottom up. If this is your type then Lava rock is the best choice of even plain kitty litter. The season is upon us happy BBQing

The lava rocks (or porcelain briquettes) are used to even out the distribution of heat. The capture & hold heat from the flame beneath them, thus producing a more radiant heat. They also protect the tubes & vents where the flames come up from dripping grease. If you don't use them, then you're going to have heat bands, and likely clog the heating source (not to mention the flames burning the delivery tubes, causing damage to the flame vents).