It's a very good question and I researched same many years ago!

Things might have changed since but the UK AIP used to say that you set the QNH immediately unless "further flight level reports were required" ie cleared altitude 2,500 QNH 1015, advise passing FL 50.

The ICAO PanOps said that the QNH should be set straightaway unless level flight above the TL was anticipated.

From a terrain point of view (esp if pressure less than 1013) set QNH asap but bear in mind ATC might want the FL passing reports.