Works in the climb, too. Take for instance China, with a transition altitude at the metric equivalent of 9,850 ft. Always have been taught to WAIT to set the altimeter until passing transition level / altitude. Main reason being, what if you are told to "stop climb or descent" prior to passing the transition? For instance you are cleared in the USA to descent from FL 360 to 8000 feet. On the way down Center says "maintain FL180 for traffic". If you set your altimeter already (esp if it is a low QNH) you may cause a loss of separation. This EXACT clearance happened to me once. Skipper had "pre-transitioned" his altimeter and I had not. He does not do that any more, as the altitude alert going off during level-off scared the bejeezes out of both of us!
Another gotcha (back to China now) is a STAR with altitude restrictions. For instance you come out of Beijing on the TONIL and have a crossing restriction at 8860 feet (or 2700 meters or whatever the heck off-kilter number it is). You are cleared to climb to a high altitude "on standard". I see a lot of people switch their altimeters AT THE POINT OF RECEIVING THE CLEARANCE. As long as you get a subsequent "unrestricted" climb clearance prior to leveling at the 8860 ft crossing restriction, NBD. However if the controller does NOT clear you to climb unrestricted, you are expected to level at 8860 feet, ON QNH AS YOU HAVE NOT CROSSED TRANSITION ALTITUDE YET!
Yes I know they don't give a hoot usually. But that's the way it's supposed to be. You transition, AT TRANSITION. That's the whole point.