I believe if you are changing your tranny fluid on a 1-2 year old car, you can pretty much use your old washer, unless it is in really bad shape. I changed my tranny fluid a couple months ago, and the washer is like brand new. No rust, no stain, not even scratched much, nor any warping on the shape. I would assume the manual suggesting that because usually people don't change the fluid until roughly 60k-80k miles, which the washers would be pretty beat up by then.
I could be wrong though.