Proper way to include plugins in your WP theme

wp-plugin-include

// This function loads the plugin.
function jz_load_plugin() {
 
	if (!class_exists('Vintage_Engagement_Widget')) {
		// load Vintage Engagement Widget if not already loaded
		include_once(TEMPLATEPATH.'/plugins/vintage_engagement_widget.php');
	}
    if (!class_exists('Jewelry_Widget')) {
		// load Vintage Engagement Widget if not already loaded
		include_once(TEMPLATEPATH.'/plugins/jewelry_widget.php');
	}
    if (!class_exists('Jz_Mini_Cart_Widget')) {
		// load my altered mini cart widget
		include_once(TEMPLATEPATH.'/plugins/jz-mini-cart.php');
	}
}