There is a great module for auto tagging nodes with tags/terms for drupal 6, hopefully something similar will be for drupal 7 aswell http://drupal.org/project/autotag
Problem with this modules is that it doesn't respect(check) pathauto setting for URL (which is non existant, more here http://drupal.org/node/1167612) so you will have lots of new URLS if your pathauto settings has changed since the first node and you have old nodes with different path aliases. So to respect this go to autotag.settings.inc and edit function _autotag_mass_update_helper($nid) so that you add
$node->pathauto_perform_alias = FALSE;
This will preserve current alias, and if you wish to mix some results, maybe try this helper module http://drupal.org/project/pathauto_persist