In and of itself, using high range doesn't hurt anything. That said, if you are creating enough pressure to activate the pressure relief valve, then you are heating up your hydro fluid. At 180 degrees, you'll start to damage components, but before you get to that temp, you'll probably notice that the tractor is slowing down for the same pedal amount due to foaming or aeration.
I would say that you'll "know" if the pressure relief valve is bleeding off fluid. It will make a different sound and you'll get the feeling that the transmission is "slipping". If you don't hear a sound or get that slipping feeling, I would keep on truckin' in high range.
According to the research I have done on hydraulics, miss-adjusted pressure relief valves are one of the main reasons for overheating hydraulic systems. This is more applicable in industrial applications that have higher working pressures and not mobile applications, but the principle still applies.