[Gasoline Fumes]

Putting a grill block behind the grill is a lot more work. Not only do you have to get behind the grill, but it's usually not a flat surface that's easy to cover either. And air pressure on a grill block behind a grill will try to push the block away from the grill and let air in. When mounted in front, the air pressure pushes the block tighter to the grill. I wouldn't expect an aerodynamic difference between the two methods if it's well done.