While they're perfectly edible, the dark green part of a leek is typically tougher, chewier, and more bitter than the lighter green and white pieces of the vegetable. Eating them (or not) is really a matter of personal preference! I recommend trimming off at least half of the uppermost dark green leek pieces: At the very top, they're pretty leathery and just not that tasty. But the lowest inch or two of dark green pieces - right where the dark green meets the white portion of the vegetable - are much more tender and easier to cook with! If you don't want the tougher texture but want to reduce food waste, you can also save any dark green leaf tops for homemade turkey stock or parmesan stock.