While it never hurts to use Auto-Rx I think it is silly and a waste to use it on a realy clean engine. If I just felt compelled to do it I with no real need to I would waite till 70,000 miles. This way you can still get away with useing just a single bottle unless you crankcase capacity dictates two bottles. You also increase your chances of getting your money's worth. If the engine is still fairly clean then you are basicly tossing money at a problem that does not exsist
If this was a used vechile with an unknown service histroy or if you abused it with extended oil changes with cheap conventional oil I could see useing it early. Even Frank has said before that deposits begin to form by as little as 36,000 miles. Their is a huge difference between harmful deposits begining to form at 36,000 miles and an engine full of deposits.