Here in France it's recommended to keep salary slips for life. Can be useful when claiming your pension if someone screwed up along the way.
Now it's easier with cloud document vaults (well, I still download everything and keep on my own server, same for bank docs). Your bank should have everything but will ask for a fee to dig it up.
on a related note, "proof of funds" will be even worse as time goes on..
ive signed messages from my mining payout addys as well as screenshots of the pool accounts that shot where my corn came from. printed it all out.. never know when it may become handy.
also satoshi mentioned (pretty sure) that keeping control of all used addys is a Good Thing.