In addition to what is mentioned in the FAQ , you will also find information on the site and in the forum with suggestions, for instance:
Hansmex' roof guide
5100 Arun's roof tutorial
4745 Problems with roof (and ceilings)
5747 Roof collection
4785 Room Elevation & Curved Boundaries
5507 Raised floor

Where you want to use levels and where you want to use "fake" floors (or ceilings) may depend on the complexity of your plan. While each level will let you define floor thickness, floor elevation and ceiling height, you also may not want a model with too many in-between levels as the plan could get more difficult to edit. ok