OP
mdshunk
That's what I've taken to doing as well. Since we're dealing with a phase-to-neutral voltage of 120V here in the states, I'll do my preliminary check at 50 or 100 volts; whichever scale the megger I'm using happens to have. The stuff I miss are things like chime transformers, roof exhaust fans in the attic, television amplifiers, plug-in air fresheners, dimmer switches, and switches w/neon lights.I tend to unplug everything then test at 250v as this at least gives an indication of anything still plugged in thats hidden away. If the reading is ok at 250v I bang 500v round the circuit.
Hidden items I've come across in the past:....
You all might find it interesting that megger checks are not required here in the US at all, at any interval. I do them myself following water damage, fire damage, or a hard lightning strike. Seems like the only prudent way to pass or fail the installed wiring following such occurrances.