I do antique mechanical restorations. Authenticity is important. That said, I replace all slotted head screws (new and old are all alike), all washers unless special made and any standard bolt with damaged threads or head.
Bolts are another story. Bolt heads in older equipment had plain heads, no load and rating markings. Any where they show, new bolts look out of place. If I can't save them or find spares in my bolt bin, I will mill or grind off the markings of new bolts.
Also there were a lot of different head profiles back in the day that can't be sourced today so they all have to be saved.
But, If it doesn't change the function or look of the finished product, life is too short to clean the rust and scale off a pile of 5/16 or 3/8 bolts that you can buy in boxes of 50 or 100.
