I wash ours about once every couple of years. Some of it by garden hose and some by pressure wash. Usually in warm weather and always avoid the electrics, steering, and seat.
In fact I cover the operator area with a tarp.
I try to remember to disconnect the battery first. That's just good practice, and a chance to scrape and clean the connections from cable to battery posts..
One of my goals pressure washing is to to get the extra grease off of pivots, and the oil spills away from leaky hydraulic quick connections, but I don't think that doing so makes the slightest difference to the tractor. I mostly do that to keep from tracking grease around.
It's good to spend some time with the pressure washer on the underneath where there are filters and hoses. That gives me clean surfaces to change filters and is when I do an annual inspect for leaks and dents.
I don't pressure wash above the lower half of the engine. There is no advantage, and just too much sensitive wiring, electricals, vents, and stuff that doesn't need to be wet. I
If I'm ambitious, I'll hand wipe around the valve cover and high pressure pumps.
The radiator fins I do by hand with a garden hose, dish soap & soft brush.