For me, certs from the period before the primary expertizing organizations had technology like the VSC6000 don't carry much weight beyond a basic ID. There are a LOT of 1990's certs that did not note reperforating, alterations and repairs that were subsequently detected upon resubmission. I have had this experience myself and a few of the stamps were returned to the auction firms based upon the new certs. Seigel, DFK and dealers like Rupp are good about obtaining new certs for pricier stamps, but
ebay and many smaller auction firms are not.
If you are buying at auction and a newer cert is not present DO place the stamp(s) on extension. If they don't do extensions, consider that when bidding.