A filter to only display Approved registrations on the EE checking page (Event Espresso -> Registrations -> Check-in)

admin_only_checkin.php

<?php //Do not include the opening PHP tag if you already have one.

add_filter('FHEE__Extend_Registrations_Admin_Page__get_event_attendees__status_ids_array', 'tw_ee_get_event_attendees_atatus_ids_array', 10, 1);
function tw_ee_get_event_attendees_atatus_ids_array() {
    return array(EEM_Registration::status_id_approved);
}