The UK Air Information Publication (AIP) states that when cleared to descend to an altitude you set QNH, unless further flight level vacating reports are required. There is nothing complicated about this and it is not necessary to even be aware of the TA/TL to apply this system. It's also much safer in terms of terrain clearance since if you omit to set QNH with pressure below STD (1013.25 Hpa/Mb) you run the risk of descendng below MSA/SSA.
It is a while since I checked but the ICAO Pan-Ops used to say set QNH as soon as cleared to an altitude unless further level flight above the Transition Level is anticipated.
Individual States publish differences in which case one should be aware/compliant etc and one hopes that Company Operations Manual clearly define how to operate the altimeters.
Finally, despite the fact that it is important to have a standard procedure for management of altimeters, it's equally important to have an understanding of WHY you are doing what you are doing with the altimeters at the time - basically, this boils down to good training.