Okay, I tried to find this info because I KNOW I've posted it dozens of times in other forums but I can't find a link so here goes.
If you are absolutely ANAL about your tire pressures, here is the correct way to find out what works for YOU and the way YOU ride. First thing in the morning, take a pressure reading from your tires and write them down. Now go for a "typical" ride of at least 30 miles or more. IMMEDIATELY after stopping, take a tire pressure reading again. If the tire pressures are between 2 and 3 psi HIGHER, congratulations... you're golden. That cold pressure reading you took in the morning is the perfect pressure for you and the tires you have.
If the tire pressure is greater, then you didn't have ENOUGH air in the tires to start. Try starting out with a slightly higher pressure and repeat the process. Not enough air in the tires allows the carcass to flex more. More flex means more heat, and more heat causes a greater increase in tire pressure.
If the tire pressure is LESS than 2-3 psi higher, then you started with too much air. Too much air in the tires means that the tire is too stiff; the carcass isn't flexing and heating up enough to provide optimal traction for you. Repeat the exercise with a little less air pressure in your tires.
Trial and error, but you WILL come up with the perfect tire pressures for you and the way you ride. By the way, if you should switch tire brands, start all over again. Every tire manufacturer and model will have different characteristics and you should adjust accordingly.
Like I said, this is ONLY if you are absolutely ANAL about having proper tire pressure. For most of us, the factory recommended pressures are adequate.