The whole system relies upon integrity; we trust a pilot to behave in all sorts of safe an appropriate ways, correctly writing down the flight time is just one of those.
I always let my engine warm up, and usually do avionics checks before taxiing. (Usually = some aeroplanes I fly don't have any avionics to speak of!). I always let engine temperatures settle after putting brakes on before shutting down. Therefore if my logbook showed Hobbs time I would be artificially inflating my hours.
G