Do I need to backup civicrm_cache table in civicrm 4.0.8
If you TRUNCATE the table, be sure you do so at a time when there are no in-process contributions or other users actively filling out a form or their submission will be lost. Otherwise, it's safe to do so.

Your backups can just skip that table, no need to back it up. Although you do need to get the structure of the table in order to be able to restore a complete db from your backup.