If the building has proper electrical with grounding, and there is heavy enough cable running to the server room (as in, it can handle enough amperage), I don't *think* you have to worry about grounding the rack itself. I don't think ours was.

Now, this is for a simple data center for a small group. If you are doing AV production, and the rack holds audio or video equipment, then grounding is something you have to be very careful about. I heard about how Skywalker sound overhauled their whole building to put in special grounding...