Kill them useless menu classes

functions.php

<?php
function pwcc_filter_nav_menu_css_class($classes, $item, $args) {
		
	unset($classes);
	$classes = array();
	
	if (trim($item->classes[0]) != '') {
		$classes[] = $item->classes[0];
	}
	
	$classes[] = 'menu-item';
		
	if ($item->current OR $item->current_item_ancestor OR $item->current_item_parent) {
		$classes[] = 'menu-item-on';
	}

	if ( $item->current_item_ancestor OR $item->current_item_parent) {
		$classes[] = 'menu-item-up';
	}
		
	if ($item->current) {
		$classes[] = 'menu-item-is';
	}
	
	return array_unique( $classes );
}
add_filter( 'nav_menu_css_class', 'pwcc_filter_nav_menu_css_class', 10, 3);
?>

Here's a simpler approach to do the same thing.

<?php
function pwcc_filter_nav_menu_css_class( $classes, $item ) {
    $classes = array( 'menu-item' );

    if ( '' != trim( $item->classes[0] ) )
        $classes[] = $item->classes[0];

    if ( $item->current )
        array_push( $classes, 'menu-item-on', 'menu-item-is' );

    if( $item->current_item_ancestor || $item->current_item_parent )
        array_push( $classes, 'menu-item-on', 'menu-item-up' );

    return array_unique( $classes );
}
add_filter( 'nav_menu_css_class', 'pwcc_filter_nav_menu_css_class', 10, 2 );