I agree with the wide open comments, although I do occasionally mow at reduced throttle on my diesel zero turns.
One thing that deserves mentioning is that most mowers I have used, particularly with electric clutches, is that the blades are generally engaged at mid throttle then advanced to wide open.
Interestingly, on my new Bad Boy Rogue zero-turn the dealer told me the manufacturer recommendation is to throttle up to WOT before engaging the PTO. He said it avoids belt/pulley flopping that happens when starting at lower RPM, which is tough on parts. Sure enough, the manual confirms that:
3.11 Increase RPM by sliding the throttle control to fast position.
3.12 Always engage PTO at full throttle.
I've always been of the mind to engage the PTO at part throttle and throttle up on other equipment, but they likely wouldn't make this statement if they didn't have a reason so on the Rogue I'll follow manufacturer advice.
I was just wondering about the shock to the belts if it's worse at full throttle or half