However the tach itself must be accurate
Well, as petty as it will sound, the hour counter portion of the tachometer will always be very accurate, as it is simply counting engine revolutions by direct drive and gears. It is the pointers which are notoriously unreliable for RPM, as they are adjustable.
I suggest that any of this is really not relevent in the context of logging piloting time though... It all averages out in the end, and that extra .5 of an hour over a year, does not mean much in the context of skill.