You can either way, but probably with a chipper I would idle down before switching the pto lever.
Just a tip guessing you haven't used a pto before always have the tractor off when hooking and unhooking the pto shaft. A tractor even at idle has enough power at the pto to grab you and roll you up in it within a few seconds. Hate to be so gruesome but many people underestimate rotating equipment.
Also try not to turn the tractor off with the pto engaged as it will slam whatever is on the 3pt to a dead stop, unless of course it's an emergency.
