Ridicule me if you like, but I always do Wheels Off to Wheels On time + 0.1 per Takeoff/Landing, so taking off at 1300, landing at 1400, would be 1.1hrs in the Logbook. Main reason is that with our paperwork the takeoff and touchdown times are the ones we record so it simplifies writing it up in the logbook at the end of a day.
The 0.1 as as i'm concerned is fairly accurate for taxiing time on average as there is usually 2-3minutes before take off and 2-3minutes after take off. If there is an abnormally long Taxi time for whatever reason then of course I take it into account, but this is very uncommon where at the places I've mainly been operating for the last few years.
Personally the way I see it is that its meant to be Start up to Shut Down time as this is the time you are "In Command" of the aircraft and, as of course mentioned, only if Start Up is intended for the purposes of take-off. If I go to take off and have a rejected take-off necessitating a return to the Apron then I'll still log it of course. IMHO it really has to do with your Flight and Duty as these are designed to reduce the risk of Fatigue Related Incidents/Accidents.