Depends on the job. For sure, you want them tight enough so the lift arms can't hit the inside of the rear tires. After that, it depends if you're going to do a lot of straight line work, or do a lot of curves and turn-around work (with the blade on the ground). If straight line work - or if you'll lift the blade while turning around - tighten them all the way up. If there'll be a lot of work on curves, give yourself some trailing slack.
I'm wondering though if a five footer isn't too much box blade for a half ton on turf tires?
//greg//