The supposed point of using nitrogen in your tires is that it is not as variant in volume with temperature, so if you inflate your tires to 45 PSI with nitrogen when it's 45 degrees Centigrade out (hi from Phoenix!), the tire pressure won't go down as much when it gets down to twenty degrees.
But since we have TPMS in modern cars, it's not as "necessary" anymore (not that, in my opinion, it ever was), it's mooted by the car letting you know when your tire pressure has changed on you.