Ok. If washers work for you, that's great. I'm not trying to sway anyone, just posting the manufacturer's recommendation.Well, there's only one statement concerning Karve's ATTENTION note that's true.
"Failure to remove the washer reduces the thread engagement."
And that reduction is less that two threads. Both my Timeless and ATT caps are cinched down tight with the washers in place with 5 1/4 rotations of the handle. Without the washer it takes an extra 1 1/2 or a full 6 3/4 turns of the handle to tighten. That extra thread engagement without the washer will make zero difference. All I'm saying is the washer will not contribute to any damage unless the threaded post is too short in the first place. In the end it will make zero difference whether one chooses to use or not use the washer when it comes to securing the cap/head to the handle.