It seems to me that the only really important thing is to make sure you have the correct setting when you LEVEL OFF. The best way to do that is to set both main altimeters to QNH/FL when you are cleared to that altitude/level. If a report or reference of the previous setting is needed use the standby altimeter.
The published or assigned trans. alt or level is really an ATC tool to allow correct separation between traffic operating on QNH and FL's. At airfields that have assigned trans. levels the lowest allocated flight level varies with the pressure and ensures that there is always at least a 1000' between the highest allocated altitude and the lowest allocated level.