From the drupal website:

Drupal 6.10 and 5.16, maintenance releases fixing problems reported using the bug tracking system, as well as a critical security vulnerability, are now available for download.

Upgrading your existing Drupal 5 and 6 sites is strongly recommended. There are no new features in these releases. For more information about the Drupal 6.x release series, consult the Drupal 6.0 release announcement, more information on the 5.x releases can be found in Drupal 5.0 release announcement.

 So it's strongly recommended you upgrade your Drupal installation.

If you have any problems doing so please contact us through our contact form or call us directly at: +31 316 843823. We can do it for you at fair rates.