It really all depends on your application.

When I use them in wok cooking. I deploy in three stages, The white bits first..then the upper bits..and finally the green bits to finish with light cooking.
For other applications...I'll save the green bits and lightly saute them and use as a garnish.

In a soup..it mostly all goes in. Except for a few garnish green bits.

Thank you all. This has been a wonderful learning for me!

If you simply chop across the leek at the outer junction of pale and dark you will lose a lot of leek.
Trim the leek in LAYERS at the junctions - the outside layer, then each underneath. The junction gets farther from the root as you get nearer the inside.
Then clean as above.
This is from Jacques Pepin.

Stand the leek upright and using a sharp knife slice down the center just down to the white part. Rinse under cold water to remove any remaining dirt. Trim the root end. Slice across the bulb (the white part) for as much as you need. The green tops can be used to make a bouquet garnie for stocks and so forth.

The white, yellow and very pale green parts are what you want when your recipe calls for leeks. I generally cut the leek in half lengthwise, then into half moons. Make sure you rinse really really well as dirt likes to hang out in between the "leaves." The darker green parts are a great addition to stock, so they don't need to go to waste.