If I manually enter terms into the _data and _hierarchy tables, is there something else I need to alter to avoid problems when I go back to manually adding terms?
Drupal 5 or Drupal 6?
In Drupal 5, autoincrement wasn't used and IDs are stored in the sequences table. You'd have to edit that table to manually push up the ID to what you need it to be.
In Drupal 6, right, this shouldn't be a problem (sequences is no more).