Jim'* right. You need to get on those things right away because they will get worse and eventually you have a hole where the rust is. Unless you have the money at hand, i wouldn't worry too much about the parking lot dings. Usually they will sand and re-paint the whole door because you will never find an exact match, even from from factory touch up paint due to sun bleaching.
Another thing to help avoid rust...it sounds like you are already doing it....on any warm days in the winter you should wash your car. If the salt sits on your car, and it warms up sitting in the sun, this is when it starts to eat at the paint. I'm rather having a brainless moment (don't even go there guys
) so i can't think of the technical term for that process.