i seen on woocommerce main report class WC_Admin_Report
on line 67
they have filter like this $order_status = apply_filters( 'woocommerce_reports_order_statuses', $order_status );
so i think you can easily alter the default order status for reports by adding filter like this on your functions.php
add_filter( 'woocommerce_reports_order_statuses', 'my_custom_order_status_for_reports', 10, 1 );
function my_custom_order_status_for_reports($order_statuses){
$order_statuses = array('completed'); // your order statuses for reports
return $order_statuses;
}
ahh, and a cup of coffee would be better than beer @gilesytheking ??