At this point, we don't shred files anymore and we shred the empty space when we have time and a motivation.
Now, the last important step is not to forget to destroy all of the data when the hard drive is disposed of. There is a lot of data that you must destroy, even if you destroyed your main "My documents": Internet downloaded files, drafts that you may have forgotten, saved passwords or connection parameters...
There are countless stories of companies being spied upon by use of their old hard drives. To get rid of this threat, you can use a hard drive shredder such as the one below.
OK. So, good practice is to establish a policy that forbids hard drives (including internal hard drives in the printers and xerox machines) going out before a shred. Don't donate, sell or dump an old hard drive before a shred.