Is there anything I should put beneath a cabinet when using it freestanding?

For a 300mm 19-inch cabinet sitting directly on a desk or worktop, small protective pads are a good idea. There are screw heads and fixings underneath that you probably do not want marking the surface.

For compatible 19-inch cabinets of 450mm depth and greater, use the feet or castors if they suit the installation.

With Miniline, make sure the bottom of the cabinet is actually sitting flat.