Sometimes cooks just do things without really thinking them through. They heard or read somewhere that such-and-such was true, but they never really checked to see if there was any basis in fact. For example, for years and years, people always said that you should remove the germ / sprout from a clove of garlic because it tends to be bitter. But when then you realize that it's not bitter, but actually kind of tasty, and all these years, you've been throwing it away.
So, what kitchen myth have you seen perpetuated that you've since found to be completely off the mark?