I usually do it before I wash the car. I take a tooth brush with a good vinyl/plastic cleaner and clean the entire thing and then I wash the car. I do it before I wash the car because the vinyl/plastic cleaner is bound to get all over the front bumper and it is just easier that way.
When you wash your car - do the grille first, rinse it off, then spray on some Simple Green cleaner. Let that just soak while you wash the rest of the car and then wash the grille again. Rinse and your done.