Thanks for the intelligent answers guys.
I've always been worried about going higher than 38 or so, figuring it's going to heat and raise pressure as you drive.
One thing which has always puzzled me is that while they always say its the cold pressure which is important. What if you do wake up in the mountains of AZ say and the pressure is just below the tire manufacturers max and then the temp rises say 4o deg F (which should be a 4psi change) over the course of the day say . Should you adjust for that?
My understanding is that the OEM pressures are at something like 60-70deg f. ("room temperature"). Since tires warm up as you drive, I'd think the 40deg outside air temp would be negligible.
Everything I've read says check it in the morning, but what if it's minus 10deg in the middle of the winter? I suppose you are supposed to add a few lbs if it is under room temp?