I've always used hours and minutes, rounded to 5 minute blocks, chock to chock. Seems to me the logical way to record data which is based on actual clock-time which is universally in minutes.
My practice is to book actual chock to chock for each flight, except for those occasions at an international airport (when the hold for IFR movements would sometimes be better measured with a calendar...) then its t/off and land each +5 mins ie flight time + 10 mins.
Adding up minutes and hours on the ground in a logbook shouldn't be a problem, after all ETAs etc are in hours/minutes, not decimals, and the brain is frequently multi-tasking at the time!
Hobbs meter is ... a meter, which stops and starts with usage, and can record whatever units its calibrated for. A clock is "kept in motion" (OED) and I haven't seen a decimal one yet.
Julian, at least there's no VAT, whether minutes or decimal-hours!
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