No, it doesn't matter how fast the engine speed is when you engage the auto-throttle - you just don't want to be moving when you engage it.
Mine was set by the dealer so that the hydro would engage once the engine RPM's hit 1500 when pressing forward on the hydro pedal. Some others here have said that their hydro didn't engage until over 2,000 RPM, which would be annoying. If you trace the cables that come off the hydro pedal, one goes to the engine throttle, and the other goes to the hydro controller. I believe that there is an adjustment nut at the engine throttle cable that comes from the hydro pedal, and with some tweaking, you can adjust it to the way you'd like it to operate.
Personally, I do virtually no
PTO work, so keeping a constant engine RPM isn't important to me. That's why the auto-throttle works so well for me. I'm also used to operating gear tractors in the past, so the auto-throttle makes the tractor handle very similarly to a gear tractor. When I need more power, I just press harder on the pedal. Very handy when working with the loader, plows, and ground engaging implements.
For break-in of most rotating assemblies, it's often better to vary the RPM so that bearings wear in evenly. The auto-throttle definitely helps with that.
Hopefully that description helps. I think most people have found the auto-throttle to be very useful when it's adjusted properly.
Chris