Well as you might know dust falls to the floor and if a computer is on the floor, it will pick up the falling dust. It's better to put it on a desk. Hints the name: desktop
Here I don't have the problem of being on the desktop or floor since the case is in a cabinet portion of the desk. In less then a week's time you will see a layer of dust formed on the fan grills for the twin 120s on the front of the 900 here. I can imagine how loaded up they would be if the case was simply left on the carpeted floor.