Flying with the Carb-Heat on for a long(er) tour?

Hi,
After taking two passengers on SF-Bay tours, I realized that I
want to take the tour slower... A north-bay tour (SF, GGB, Ocean
Beach, without the 101/Bravo transition) takes about 40min roundtrip
from OAK - a little quick.

I've been used to flying with the rpm around the bottom of the
green arc for this kind of thing, as my CFI used to always remind me
that "Carb-Heat introduces unfiltered air [into the carb]" which is a
bad thing...

Basically - is it bad for the aircraft (C172) to float around at
around 85kts with the power somewhere below the green arc (say
1800rpm) and the Carb-Heat on? Ok, there is a second benefit - my club
charges based on tach time, and I have to be honest about that -
although I really do wish for the tours to be slower.

Thanks for any comments,
Noah