Close some meta boxes by default instead of hiding them altogether

functions.php

<?php

add_action( 'admin_init', 'my_close_meta_boxes', 99 );

function my_close_meta_boxes() {
	$post_types = get_post_types( array( 'public' => true ) );
	foreach ( $post_types as $type ) {
		// Close Excerpt on all post types
		add_filter( "postbox_classes_{$type}_postexcerpt", 'my_custom_closed_meta_boxes' );
	}
}

function my_custom_closed_meta_boxes( $classes ) {
    array_push( $classes, 'closed' );
    return $classes;
}