Revisiting this thread because I was swapping out cassettes and finally remembered to take a picture of the crush washer I was talking about...
This is the one from the cassette that had been sitting in the toolbox for the last 3-1/2 months. As you can see, the deformations have not relaxed even though there's been no torque on it for that period of time.
I re-installed it with that washer, but it still makes me nervous.

Also - this can't possibly be related, because the cassette I took off had only been installed once, with the new crush washer - but it was loose when I went to remove it. Not loose enough to rattle, but loose enough that it took very little effort to remove. I'd used my good Snap-On torque wrench to install it, as always, so it wasn't undertorqued. Why would it be loose?
Speed comes from what you put behind you. - Judi Ketteler