My soon to be replaced computer sits on a homemade 2 1/2" high skid with castors on the bottom, for easy access to the back side panel. Most of my junque is homemade... don't even get me started about my dual printer desktop stand.
My wife's new Conroe box, which is my first build, sits on a stained poplar plank on the bottom of a steel framed corner type computer desk. Both computers are over carpeting, but the wife's machine will be over hardwood as soon as I can get it changed.
When I finish my own new Conroe Christmas gift, I'll consider building a wall mounted swing arm hanger for the box, to get it higher up in my little computer room. I hate being unable to access my computer's guts... same for TV, sound system, and satellite boxes. I don't like the dust, but difficult access is just unacceptable. At my age, higher mounting means easier access, so higher is always better.