tbh my area is qute open and when i ride on roads not overly busy. the main problem i had was that i'd hear a car and check over my sholder and it'd be on another road entirely or when swinging my head i'd drift into the middle of the road :S. so now i use headphones and just keep left(UK).

The cars i would hear that might hit me wouldn't make any difference and if i ever see them parked i leave a note under the windscreen wipers...