This is the recommended way to add a custom or second style sheet for child themes:
function custom_style_sheet() {
wp_enqueue_style( 'custom-styling', get_stylesheet_directory_uri() . '/css/custom.css' );
}
add_action('wp_enqueue_scripts', 'custom_style_sheet');
You can also add style sheets conditionally http://wpsites.net/wordpress-themes/second-style-sheet-theme/