That conical area that contacts the injector near the pintle gets flattened. Compare an old vs new and you will see the old is "mushed" or "squashed" by the injector. Sort of like a lock-washer than gets deformed after you use it, loosing some of it's "springyness".
A friend who really knows his VW Diesels (thinking about it, I should get him up here on this list!) had a little trick of hammering a cone-shaped tool to peen the edge back into place. I have tried it myself and sometimes it peens over to the side. So I guess I might use the ones that have been re-peened and looked still stragiht and not lopsided, but I just try and keep a stock of new ones and use them instead, which I trust more. The peened old ones are more of a last report IMO. I would not use old ones because if they don't do their job and seal, you will get a leak and cabon will build-up in the head behind and in the injector threads.