will answer your 3 questions in order;
white grease pencil or white stick from brownells or the like, ensure it is not going to damage the bluing; added by owner; does not affect value because it can come out.
Some owners like it to see the markings better for pictures, know a few collectors that hate it and remove it. ONLY added whiting from the factory is in the Geishert / safety area, this was put there by the facory and is a paint material.
There are a few postings like this on the forum, somewhere...
Ed