Every flight school I have dealt with has used only two separate timers for their maintenance and student (or hire & fly) billing.
For charging of students and hire & fly customers it was always hobbs time (engine start to engine stop).
The time recorded in the MR for maintenance purposes was always airswitch time (even on aircraft which also had tachometer). I have flown G1000 C172 aircraft which have the tacho time on the G1000 panel but we always recorded airswitch and hobbs only.