I was a very avid knitter for a while. I still knit, but not as obsessed as I used to (but that can happen again). You are usually advised to keep the labels from the yarn you have knitted with, for later reference.
But you know what? I have never checked them. When in doubt, you can wash all wooly items by hand or with the wool-program of your washing machine. And I documented my knitted things on my Ravelry page.