A bit harsh Beagale. Use of an appropriate grade of oil or a multigrade prevents wrecking your 15K engine, many people believe that sitting at idle for any length of time after start up is detrimental to engine life (actually it is start up and shut down that knocks the hell out of ya engine). Waiting for hydraulic tappets to 'pressurise' is beneficial but this doesn't take long and operating temps will be reached quicker if you are using power settings appropriate to taxi speeds and if not, your engine run will soon take care of that.
I don't suggest taking off immediately after engine start but I don't recommend hanging around too much beyond your after start checks before doing an engine run.
I subscribe to the when in Rome theory, I have always logged hours based on how I am billed. If it's Chocks Off to Chocks On plus a bit or minus a bit that’s how I do it or if I am billed on the Hobbs that’s what goes down in my log book. If someone wants to check up on me there can be no question.
I believe Hobbs hours are based on engine rpm so theoretically this should be the fairest way to log hours. Airborne time is minute for minute but if you are sat on the ground nattering to your instructor waiting for departure, you aren’t credited with that entire period of time.