Hmm. I've heard conflicting opinions about whether to shut the car off or not. I was pulled over a few months ago for ign'antly blowing past a Charles County trooper on a cold night, and I left the engine on so the heater would keep running. The weather might've made that an exception, but is it really significant to turn off the ignition?
In talking to my LE buddies (IANLE), turning off the engine makes them feel slightly less apprehensive since you have limited one variable from the equation (movement of the vehicle for escape/evasion or running the officer down).