If I'm making something with a long cook time (like some oven braised short ribs) or with a blending step (like this leek and potato soup), I'll leave a few inches of the dark green layers at the top of the leek. I like the more complex flavor those pieces bring, and they'll be sufficiently broken down during the cooking process. If I'm cooking leeks for something more delicate - like a creamy pasta sauce, a leek frittata, or some leek breakfast hash - I'll often trim away more of the green pieces and use just the white portion of the vegetable. I'll also use just the white and lightest green pieces of a leek if I'm roasting them for something (they cook more evenly that way!) But the question of "how much green to leave" when you cut and clean leeks is really a matter of personal preference: So if you like the darker pieces, leave them - and if you don't, trim them off!