If you get higher grade led's you dont have to worrie about voltage/ current spikes, i use these in a couple cars and they have lasted well over a year so far and still going strong.
These are the ones i use and the site i recommend.
http://www.superbrightleds.com/cgi-b...2FWLED-xLX.htm
They are non-polarizred and are rated to 30 v rather than cheap ebay ones that are rated at 12v.

Nearly every bulb in my car is Led, i dont use a resistor box, just good quality Leds.