Already used in Formula One, Military Aircraft and Space Shuttle tires, nitrogen is the safest, best possible tire gas.  Tires inflated with nitrogen run between 10 and 15% cooler.

Tires inflated with nitrogen do not lose pressure as quickly as those filled with air. If there is no damage to the rim or tire, the pressure will remain constant, because air migrates faster through a tire than nitrogen.
 

Recommended tire pressure Tires inflated with nitrogen should be inflated to the same pressure as air.
i.e: the manufacturer's recommended tire pressure for the vehicle.
 

You can top up with air. Air is already 78% nitrogen, mixing the two will not cause damage to the tire.
You can re-inflate to 100% nitrogen at your convenience.