If I understand the question correctly, It is designed to work either way, that is why the rollers have an open bottom. There are a few builds that do it that way.I chosse to build mine with an open system so I can mount the material any way I want. Besides 3D printing on it everything else hangs out at least 1 side, you can see it in my videos, I hang it out the side and clamp it down. To me it is easier that trying to just screw it in to the surface board.
So ease of use, and but you can do it that way if you want. I tried to build this machine to be as configurable as possible (some design compromises had to be made).