Set a default category – if the user didn’t select one- before creating a post

Maybe this is what you are looking for?

Source is from @Michael Fields

/**
 * Set default cat for cpt
 * @source {https://circlecube.com/says/2013/01/set-default-terms-for-your-custom-taxonomy-default/}
 * @source {http://wordpress.mfields.org/2010/set-default-terms-for-your-custom-taxonomies-in-wordpress-3-0/}
 * @license   GPLv2
 */
function set_default_object_terms_203962( $post_id, $post ) {
if ( 'publish' === $post->post_status ) {
    $defaults = array(
        //'your_taxonomy_id' => array( 'your_term_slug', 'your_term_slug' )
        'post_tag' => array( 'taco', 'banana' ),
        'monkey-faces' => array( 'see-no-evil' ),
        );
    $taxonomies = get_object_taxonomies( $post->post_type );
    foreach ( (array) $taxonomies as $taxonomy ) {
        $terms = wp_get_post_terms( $post_id, $taxonomy );
        if ( empty( $terms ) && array_key_exists( $taxonomy, $defaults ) ) {
            wp_set_object_terms( $post_id, $defaults[$taxonomy], $taxonomy );
        }
    }}
}
add_action( 'save_post', 'set_default_object_terms_203962', 100, 2 );

This works like a charm for us.

ps, there is also a plugin which could be helpfull to ‘force’ some of your wishes before an user(read editor/author etc.) even can publish.