Mike, I assume you are talking about the WM and does it have a hydro transmission? Hydros run at 3/4 to full throttle most of the time to be the most efficient. Lighter loads don't necessarily have to
Any PTO work like bushhogging requires the PTO at 530-550 rpm range to keep the blade tip speed at the correct speed for smooth cutting. This varies tractor to tractor. The PTO is engineered to run around the peak of the torque curve. On some tractors this could be close to full rpm. Whatever the case, you aren't doing your equipment any favors not running at the optimum speed.
Chiseling and disking full speed is fine, but again, you need to run at the upper end of the torque curve which could allow you to throttle back a little.