Hi Sims it sounds as though your layer are turn off and on according to the way you want to view your model. Simply go to your layers menu in Skethcup and select the way you want to view your model. The reason why we do ot cut out door frames is because you or the builder will actually need to buy this timber to be able to construct the project.
The best tip I can give is to use the scene creator
and use the scenes as they will do this for you automatically.
Good question mate.