I would imagine that in a world composed of law and goodness, trade would occur in benefices and indulgences. So 'tipping the cherubim would mean leaving some of the superbulous virtue you accrued from one of your good deeds with the guardians of the gate, so that they could mercifully endow someone who was just shy of the necessary amount of virtue with your goodness and allow them in. In this way, virtue would multiply in the trade much the same way that money multiplies, in that leaving the tip is itself a good deed and creates goodness that spreads around.
I don't know how much Rich has thought through this or whether it is just a punch line, but nothing would surprise me at this point.
Pour boire du vie.