Disable all notice errors on a drupal 6 site
Its not so much a Drupal 6 question, but a PHP question.

In your settings.php file I sometimes do the following on my Drupal 7 sites:

// Drupal Bootstrap is overriding even htaccess values for this. I want the
// notices but not for them to display in production.
ini_set('error_reporting', 'E_ALL ^ E_NOTICE');