Don't even worry about nitrogen. It's important in race tires where the tires get pretty hot and the difference in pressure between air and nitrogen actually matters for consistency and grip. Somewhere around 1-4 psi difference at race tire temps is common, depending on lots of variables. Your tires will never get that hot on the street. The common knowledge about the difference in pressure at street driving temps is less than 1 psi (if at all under normal driving), and at that range it's tough to narrow down any differences in alignment, vehicle weight distribution, etc., all of which can have a larger effect than the type of gas in the tire. And you don't know the moisture content of the air you're putting in, so you have no idea what you're actually comparing the nitrogen to.
And I know Porsche and some other manufacturers use it from the factory. This is probably because of a few things.. First, people like to think that they're getting something special for their money, and if that means the same type of inflation gas that race cars use, fine. Second, you can generally neglect tires filled with nitrogen a bit more because nitrogen seems to seep out slower than air. Third, it makes for really low maintenance tire filling equipment because it's dry, clean, and largely inert.
So just check your tire pressure often and you'll save a lot of money on nitrogen fills. If you check your tires like you should, you'd need a bottle at your house or you'd spend a lot of time at tire places paying them to do it.
Oh, and even though it's heavily marketed, most of the stuff you hear about nitrogen is either exaggerated or doesn't apply to street driven vehicles. People say stuff like "they use it in plane tires, it must be good!", but don't think about the fact that planes have a shitload of weight, very high speeds during landing, really huge temperature swings when they land and fly, and aren't supposed to use flammable gases. That's my favorite marketing-hype example, sorry. So yeah, nitrogen is great for planes. Does your car need the same treatment? No.
It makes money for a lot of people though. Don't buy into the hype. We use it in race tires, and have a few cylinders full of it, but I don't even bother using it in my street car tires even though I can use it for free. It just doesn't have any benefits on the street. Whenever the weather changes a lot, I just adjust the pressure for free at a gas station when I leave the house. I'd have to do the same thing with nitrogen, and it's a hell of a lot less convenient.
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