Ya know, I think I'll chime in. So far I can say that I agree and disagree with what has been said.
However,
I once had an instructor who would pat me on the back after the flight, walk out the door whilst saying, "see ya on Thursday." and then charge .5 for a post flight brief, Uhhmmm, Yeah, HOMIE DON'T PLAY THAT!!! He lasted 3 or 4 flights, ohh and he was a crappy CFI. Also, I was forced to fly at a particular FBO for college credit for my instrument rating, these people were known for overcharging and boy oh boy did they. Maybe 1.2 or 1.3 ground would always be 2 hours of billing. I was red faced mad about it a couple times, not to mention that my instructor took me for a SEVEN hour jont all over hells half acre for my long IFR XC. Later I heard that he wanted XC time to meet IFR 135 mins. Just the other day, (now that I'm a CFI) I learned that another CFI picked up a flight with one of my instrument students, and guess what, took him for a 3 hour XC when they were supposed to be in the practice area doing BAI. It's really a shame that some CFI's tailor their logbook to a students flight lesson, doesn't help the reputation of the industry, or that particular individual.
When I was freelancing, I wouldn't hound the clock, I recall many time spending 1.1 or 1.2 hours ground with a student and only charging 1 hour.
Unless you are there with the student giving (ADEQUATE INSTRUCTION)
wait for it
wait for it
DON'T BE CHARGING A DAMN THING
whilst you drink a cola or drop a deuce!!!!!!!!!!!!!!!!ONE!!!!!! x three