That's the thing, Car manufacture recommends 32 for comfort...yet most of the tires I've owned have a max of 51. Now, I'd never run at 51, probably loosen every bolt in the car. But there really isn't much of an issue running from 35-40 cold if the tires are rated well above that. Thad said, traction will typically suffer the higher you go. So if you're at 42, expect issues in tight turns, fast takeoffs or even wet weather; also will likely get you stuck in thick sand.
Me personally I'll check pressure late afternoon after a drive, let tires cool out of sunlight for about 20-30 minutes, then check. Keep that at 36, which leads to a cold morning of about 33, hot run of about 38-40. I make major adjustment about 3 times a year, end of summer, middle of winter, and late spring, dependent on constant temperatures at the time.