The only way I would ever flush my tranny is if I had it flushed from new at regular intervals. Flushing can kick loose any debris built up and damage or prevent the tranny from performing properly. Heck, I drain and fill my tranny once a year. Excessive ? Yes, But while I am under the car changing oil, It;s easy and is a great piece of mind. Don't forget to replace the crush washer on the drain plug.