This advice was recieved from the instructor several years ago when I took my initial courses-
Always, always, save your old files. He claimed he made money from them on a regular basis. I have not quite figured out how he 'made money' from them, but when working with my old boss, the old files were invaluable. Many times out doing inspections on homes he'd done in the past it would save us tons 'o time measuring and gathering plat maps. In this manner, the old files saved money. You could also tell right away if there had been later additions and it would remind you to start asking about permits and such. Many times I was holding an old file that was well over the 5 year requirement.
Then comes the rare but possible case of getting pulled into court regarding an appraisal that another may have altered after it left your office. If you pull your original off electronic data and print it, looks new, right? So it could look as though after being notified of the pending litigation, you could have altered the data to avoid prosecution. Maybe the lender didn't falsify the data afterall??? If your appraisal appears as an aged paper copy with the original data that would support your innocence, the cut and paste job the end user did to falsify information would be much more apparent. Rare as this may seem, it did make sense to me and I've printed every file I've completed just for this reason.