Lice like to hide in little cracks around the coop, so I would dust the perimeter and maybe the roosts/nests if they look like they need it.
However, I never use dust on my birds because it is too much of a pain. I use Adam's flea spray, it works great and is easy to apply. Just spray under the wings, around the vent (be careful not to get it IN the vent though), and under the wattles if you feel any lice/eggs there. The only thing to be careful about is separate the roosters for a few hours from the hens after spraying. Breeding while the hens still have spray on their vent may cause sterility in the rooster (I've never had this problem, but I have heard of it happening).
However, I never use dust on my birds because it is too much of a pain. I use Adam's flea spray, it works great and is easy to apply. Just spray under the wings, around the vent (be careful not to get it IN the vent though), and under the wattles if you feel any lice/eggs there. The only thing to be careful about is separate the roosters for a few hours from the hens after spraying. Breeding while the hens still have spray on their vent may cause sterility in the rooster (I've never had this problem, but I have heard of it happening).