The Tips in detail

 "If you start building complex scenes (like "My Room" or "Sat Dish"), use several files."

You should design each object in an individual file and merge them later into the final scene. The reason is, that huge face-counts easily wrestle your system to its knees. Consequently huge scenes are not editable comfortably - if at all.

When designing objects in separate files you are less likely to mess up a whole scenery if you make a mistake. Further, you can reuse objects later much easier in other scenes.