If you get cleared for the approach above the transition level, would you wait to cross below it and swith to Baro reference, or as soon as you are cleared for the approach make the switch?
Getting cleared for an ILS approach from a Flight Level is common where I fly. We keep STD (or QNE) until intercepting the glideslope beam, then we set QNH on all three altimeters and complete the Approach Checklist from there. Yes, it becomes a bit backwards since Approach aids is part of that checklist
To me the most logical is:
Cleared to an altitude - All altimeters set to QNH
Cleared to a Flight Level - All altimeters set to STD
That said, there's traps involved like a stop climb or.. hikoushi had some examples from China.