Car USB adaptors are far superior to the 7805 solution, as they're pretty much all switchmode devices (use less power, and get less hot). Additionally, designing a car power supply is trickier than you expect, because the '12v' supply is quite noisy - *especially* during ignition. Therefore proper smoothing is an important consideration.
I tend to have my usb adaptor unplugged when I turn on the ignition, just to make sure - although I've forgotten occasionally with no ill effects.