Firstly, you can set the minimum number of tags in the options (Admin > Posting), so that would ensure they entered tags.
But if you don't want to force the user to add tags, a Filter plugin would probably be the best for this. Use the filter_question method and do something like this:
filter_question(&$question, &$errors, $oldquestion)
{
if (count($question['tags']) === 0) {
$question['tags'] = array('tag1', 'tag2');
}
}