We must be in the minority - we don't use the Hobbs or tachometer to measure time, we use a clock - and as a result don't get people trying to fly with stone cold engines! We charge for 'Time from starting the taxying process with the intention of taking off to completion of the taxying process after final landing rounded to the nearest 5 minutes'. I.e. you pay for what you may log. Instructor remuneration is based on the same time.
So we lose a bob or two. But it makes calculation of charges easy - any disputes and we just check the pilot's personal logbook!
The odd Spam-trained pilot or Hairy Arm Corps refugee who use decimal time have to be re-educated to use 'real' time, but everyone else finds it a very simple system!