On my 755, my understanding was to get the best out of it, that the high PTO or 3200 something RPM is where it should always be run at. The only time to drop the rpm was to engage the PTO.
I apply this to the 3520. I run it at the high PTO mark on the tach, and drop it to the low RPM mark for engaging the PTO.
Going uphill is putting a load on the system. If you are going up and down try leaving at the high RPM PTO mark. You're not hurting the engine any.
I've run my 755 15 years that way and the 3520 for 3 years now the same.
Lugging the engine is not good. When in the higher ranges, IMHO you should be at the high PTO rpm.
If you want the low RPM, then run in the low slow range. It only takes a few more seconds off your life when passing your neighbors house slowly.