My understanding is that ZT front tires can be pretty full of air... don't really want much deflection from them as all they do is keep the front of the machine at a constant height. The rears would probably be happiest with the least amount of pressure you can manage, as long as they don't deflect much and change the height of the deck. If they deflect when you sit on the machine, they're probably under inflated. Over inflated rear tires will probably result in less traction for steering and propulsion, and under inflated rears will improve traction but might negatively impact cut quality. Just my .02
When in doubt, follow the owner's manual. The pressure listed on the side of the tire is the maximum pressure that the tire is designed to handle, not what is optimal for the machine it's mounted on.