If you suspect the screws have any rust then I would replace them straight away.(
Good point. I would replace all
of them if I was in your shoes...If one screw is indeed worn out, weakened or on the brink to break, you have no clues about the good health of those remaining in place without showing full-blown symptoms of rust. It's called preventive maintenance.
You might want to check the following thread for a good discussion on rusting screws and how to clean the brownish stain :