We use nitrogen in the circle track stock car. Using it on a car that sees open track use may make sense, but there is no need for it on the street. Air VS Nitrogen, both hold air for a long time. Nitrogen does not expand, causing the tire to grow in diameter when it gets HOT. Street use does not heat the tires that much. Circle track tires can see over 250 degrees, and street tires aren't likely to exceed 180 degrees, plus a street tire is thick and less likely to "Grow" than a racing slick.
As such Nitrogen won't hurt, but won't much much of a difference either. If they don't air them up (Using Nitrogen) with a line that has a dryer, then deflate them again to flush out the air and moisture, it will have even less of the slight effect.
Not worth it for street use to me, but to each his own, do what makes you comfortable!