In a street car, it's not really worth it. The air we breath (and therefore gets pumped into your tyres) has quite a lot of Nitrogen in it anyway, so if they're making you pay for it, don't bother.
The main reason they add Nitrogen to tyres is so that the pressure stays balanced over a greater range of temperatures, so if you're not going out to regularly track-thrash your car, the tyres you're using will not really require it. With street driving, just remember to always check your pressures when the tyres are cold, and if you can't do that, then allow an extra 5psi or so over what your cold temps should be...