I've always been quite suspicious of the "Flush" that's now offered. On my '04 I used to change the filter... and it was a bizarre unit that actually would filter the fluid. Not a screen. Thing was, I recall it was an insane amount of fluid that the trans would take. Like 14 quarts... only problem was that they no longer put drain plugs on the torque converter that year... so the most I could change was what was in the pan.
So... that's what I did. Annually. Which ended up being every 35k miles.
BTW... my wife had the "flush" done on her Z24 at 40K miles. Thank gawd for "extended warranty" as we had it back two weeks later replacing a blown trans seal.
So yeah.... I'm not sold on this "flush" thing at all.