Use low or medium, not high. Loading the hydraulic pump and hydraulic motor unnecessarily will create overheated hyraulic fluid and premature wear. Use the lower ranges to keep lots of fluid moving through the system and the oil cooler.
You should run your engine at whatever rpm gives you 540 pto rpm, (per your ops manual), mine is 2200 rpm in 1st pto range, then set your ground speed at whatever feels good.
I agree with Neal. The lower range you use, the less pressure on the hydro is required. That translates into Less heat build up and longer life. You will also notice much improved power on hills.