It is recommended that you back up the KM server data files (for example, Security\Mgrent), separately from other data, and that you make sure these backup tapes more secure than your everyday backups. All keys in these files are 64-bit CAST encrypted, so this database is extremely secure. Remember that this database contains the private encryption keys for every user in your entire organization.
The problem with tape cartridges is that they are maintained offline. If someone were to steal one, that person could restore the files to his or her own server, and then try to crack the key used for the database, with no fear of detection due to online logons, and so on. For more information, see the Microsoft Exchange Server Administrator's Guide.