For me, it was: “If it’s going to help your players have more fun, cheat. Fudge a die roll. Make shit up. The dice don’t tell you what needs to happen, your players’ reactions do.”
Obviously, many people will disagree with this, but I’ve always appreciated this advice, and I believe it has made me a better GM.
Yeah, it is probably important in WH40K more than D&D. At least from what I have seen of the game when I would go to the local game shop.