We log flight time. I usually forget to note it so I note the time when in flight from the GTX330 capable of showing the flight time. If landing without proper notes I log the flight time from the transponder. Logging Flight time gives less hours on the aircraft. For my personal log book I add app 10 min in total - some for the takeoff and some for the landing to make it fit with 6 min increments.
All renting on our aircraft is paid on tach time only.