Just FWIW, my transmission guy said the same thing. If the history of transmission service is unknown or known to be neglected on a higher mileage vehicle, then its best not to flush. He said he would do it if I insisted, but would not take responsibility if it need a rebuild shortly thereafter. To his credit, he would have been the guy getting paid for the rebuild job anyways, so he potentially lost some paid shop-hours. All he does is transmissions, and is highly regarded.