I'm not sure if I ever updated my thread after painting my valve, but click the link in my sig and scroll to the end. I think I showed some pics of the rusty valve. I used it once and left the tractor out in the rain. It pretty much flash rusted and then I had to sand and wire wheel the whole valve with a tiny dremel wire wheel. It was a PIA!
You definitely should paint it. I sprayed mine with rust inhibitor, then two coats of primer, then 4 thin coats of kubota orange from my dealer. I masked off the valve very carefully as to only paint what I wanted to paint. I used cardboard and trash bags to block any (well, most) overspray.
If its a stack valve, and not all hooked up yet, it would be a lot easier to mask and paint.