The only cars that I delete are those really very small, but that's rare: I usually wait that at least one other admin puts a message so I see that I am not the only one to think that it is too background.
Sometimes it is hard to guess from a picture if the car is worth mentionning or not for the people that didn't see the movie. I remember that one for example: http://www.imcdb.org/vehicle_20573-Cadillac-Seville-1978.html