Both clubs I've towed for we run downhill at 2400 RPM and high speed, 120-140 mph in the pawnee. I fly my final with the throttle at idle, and judging by the steepness of the approach of my fellow tow pilots most of them do too. Our engines typically go to TBO, no cracking. One thing that helps them get to TBO is they are flown a boatload.
-- IFMU