Depends on where you want to go with the restoration and what kind of car it is. There are some cars that have unique flat washers that generic ones would stand out under a concourse scrutineering. If they are just flat and not thicker or thinner than the generic, than there is no reason to replace with expensive ones. One of my old Italian cars used locking flat washers with flat edges. In my business, which is restoring scientific instruments, I recycle all the hardware I can from scrapped units and upgrade with stainless where possible. No extra points for using original parts in these restorations.