It is important to know that the recommended tire pressure for your vehicle (as specified in the owner's manuel and the tire placard for the vehicle) are both based on cold inflation pressure.  This means that the tire pressure should be checked in the morning before the tire has been run, before the ambient temperature rises during the day, and before the tire is exposed to direct sunlight.

A good estimate to use when comparing tire pressure to air temperature is for every 10 degrees F, tire pressure will adjust by 1 psi.  For example, if the outside air temp increases 10 degrees, the tire pressure will increase by 1psi.  Conversely, if the air temp falls 10 degrees, the tire pressure will decrease by one psi.

Bottom line -- you should always check your tire inflation pressure when your tires are cold, and inflate them to the recommended pressure at that time.