I currently have an unplated car sitting outside while I get some administrative tasks done. Should I leave the screws for the license plate in?
It is my OPINION that you should leave the screws in place, even without plates on the vehicle. If for no other reason so you don't lose them while the vehicle is stationary. They won't rust any more than if they were installed with plates on the vehicle, so that shouldn't be a worry. Depending on the vehicle, most have nylon inserts which the screws go into, so no worries of rusting there. Even if it has metal screw inserts, it will be just as exposed as it would be without the screws in place. Realistically, there is no real reason not to leave them installed. As I said, though, this is my opinion as I don't think you'd get an answer which is not opinion on this one.
You don't mention what state you're in (and you didn't ask), but in many states you cannot leave an unlicensed vehicle on the street. That said, I would leave the screws in, as that would help prevent them from getting filled with sand, dust, small debris, and oil. It's probably worth mentioning that if you're going to leave the car in the environment for any length of time (> 6 months?), you probably have more to worry about than whether or not you should leave the license plate screws in or not.
Like Paulster's answer, this is an opinion-based answer. I'm not sure you could get a fact-based answer.