I used to inflate to the max listed on the tires, but on someone's suggestion (may have been an auto shop teacher; I don't remember), I started keeping the tires right at where the rating plate (or in the Benz's case, the fuel door) says they should be. So, in the 300D, I keep the rear tires at 32 and the fronts at 28, and on the truck, I keep the front tires at 50 and the rears at 80. Gotta love load-range E tires, eh?
Now if I could find a 12v air compressor that could get tires to that kinda pressure...