I don't think they did in the past. There are enough labels that are wrong to prove that. But they have set a precedence, and let's face it, a business in graded Modern's around what the label says. When do you decide what's important and what's not? Say a character gets hot in 5 years and then they start labeling it. There will be 5 years of books that don't have it on the label.
Look at the amount of information on a Golden Age book. There is not near the same amount with Modern's. They label "Joker Cover"s. Yes, I can see Joker is on the cover of the book I'm holding, ha.