Storage Policy and Retention Schedule....There IS a difference! | Computerworld Blogs:

As it gets easier and easier to store larger and larger amounts of data, we have to enforce a certain amount of organizational discipline around what we store, where we store it, how we file it, and how we know when we no longer need it.

Ultimately, these polices should be handed down from on-high, but in the real world, mane members of upper management don't think about such things.

For audit purposes, you need to keep some sort of trail for important documents, some way of verifying that they've not been tampered with, and some way of ensuring that they can't be deleted before they're no longer needed.