Car electrical systems are a really nasty environment: lots of annoying noise and voltage variations, but also potentially-lethal spikes.
If it's convenient to run off the PC's 5V supply, I'd do that, and rely on its protective circuitry.
If it's not, I'd add a protective circuit to the power input.
Put a 1N4000-series diode and a resistor (about 10 Ohm 2W) in series with the power. Add a 27V-or-so Zener to ground to clip spikes, and a .1uF cap to filter some noise. If you don't have a fat electrolytic on the power input of your Arduino, add it to the protection circuit.
Ran