People are stupid. Just check the air pressure in your tires regularly, and you won't have the loss of mileage that you get when you're running around in an Explorer w/ 12 psi of pressure in the left front.
Also, the *only* benefit of nitrogen is that the pressure increases more linearly than wih plain air due to the water vapor in air. But since we live at 4000+ feet and have almost no humidity, it's not like there's much difference here. Plus, linear pressure increase is only a big deal on the race track... air pressure shouldn't go up enough on the street to really even notice.
(IB nitrogen made my car roll.
)