D of B - that's certainly what I teach!
If ATC clear you to "Descend altitude 5000 ft" from FL wotever and subsequently ask you to advise passing FL something else, that request should be made with sufficient time for you routinely to reset SPS; surely they should assume that you have already set QNH in accordance with your clearance?
Similarly Transition Level is surely the latest point at which QNH should be set during descent, not the defining point?