When working with the WordPress Customizer, you may want to be sure a theme mod gets set on save. Unless a setting gets changed in the Customizer, it's values do not go into the final theme_mods data. This object snippet will do just that for you. The Gist includes both the function to use and the hook to attach it to for checking on save.

set_default.php

/**
 * On Customizer save, checks for and sets a few
 * default settings.
 * @param \WP_Customize_Manager $wpcustomizer
 */
public function set_defaults( \WP_Customize_Manager $wpcustomizer ) {
    $mod = get_theme_mod( 'my_theme_mod_name' );
    if( empty( $mod ) ) {
        set_theme_mod( 'my_theme_mod_name', 'x' );
    }
}

/**
 * Customizer constructor.
 */
public function __construct() {
    // Additional code awesomnezz goes here. 
    add_action( 'customize_save', array( &$this, 'set_defaults' ) );
}