I know the internet has some answers about this topic, but I'll feel more confident (and hopefully I'll finally shred any excess paperwork) if I hear the details from you smart people.
I think I have the majority of the quarterly statements from all our investment accounts going back to the 1990's. When I started, someone explained that I would need the initial investment and reinvestment information to calculate basis when retirement withdrawals began (or some such accounting). I thinks the investment firms used to ask if you wanted fifo/lifo accounting. Somewhere along the way I think the rules changed, but my behavior didn't.
Do you keep quarterly or yearly statements? When you retire, do you have to worry about accounting for capital gains within your 401k or IRA withdrawals?