At lower revs it under records, at higher revs it over records. As long as it doesn't under-record for the whole of the flight than that's ok. But if it records some amount of time less than the true TIS (as defined) then it isn't legal.
It is true that the tach will under record at lower RPM, however, not much, until the RPM is so far below the 1:1 that the plane will hardly maintain altitude. As the tach and hour meter are a part of the approved type design, they are required to meet accuracy standards. In Canada the standard for accuracy is +- 5%. Therefore your tach time could be 5% out either way, and still be legal. I cannot imagine an inspector declaring an unacceptable situation with TIS being recorded from an airworthy tach hour meter.