So far as I know A CORRECTLY WORKING standard Hobbs meter (tacho) records actual time irrespective of revs.
Having said that, I used to fly a difficult to start syndicated aeroplane whose Hobbs would jump during the starting cycle. My record was 1.3 hours on the Hobbs until I got the engine running - actually about 10 minutes of mucking about and swearing loudly at the aeroplane.
Strangely I don't fly that aircraft any more, but noticed it for sale on a notice board the other day.
Looking at a couple of online catalogues such as
http://www.wicksaircraft.com/gotopage.php?page=170 , or
http://www.ishams.com/p1000afms.html I can't see any reference to scaling.
I suspect that wise renters would rent dry, then there's a definite incentive upon hiring pilots to fly at best economy, which will tend to be treatment that prolongs the engine life (well except in the first few hours after a new engine has gone in anyway).
G