When at the table should you place your napkin in your lap?

Proper etiquette suggests it is most appropriate to place your napkin on your lap the moment you are seated.

However, it is also appropriate social convention to wait until your host or hostess is seated to do so.

In the same vein, it is also appropriate not to pick up your cutlery to begin eating until such time as your host invites you to dine, such as saying 'Bon Appetite.'