I've had lots of incidents like this that I'm sure I'm not expected to report- e.g. opened car door and slightly banged lamppost with it, dead bird fell out of tree onto bonnet, went over a pothole and engine cut out, banged into kerb when doing 3 point turn, minute slight scratch to bumper caused by shopping trolley, etc.
However I'm not sure where the line should be drawn. Perhaps it's if there's the potential for another party to put in a claim- e.g. if you throw open your car door and put a dint in their door and they make a note of your details