Use levels if it concerns placing walls and rooms (floors). Other elevations you can do anyway you want. You can also use two levels with the same elevation which allows you to make one invisible. For example one with the walls and floors and a level at the same elevation with the furniture.

You should read my manual Levels Explained .
With two houses with different level elevations the number of levels can get high fast. You might find the plugin Level Tags Manager helpfull

Edit: typo