I carry both ways. Sometimes because the knife only allows either tip-up or tip down, sometimes because I prefere a certain knife a certain way.
With the Leek I definately prefer tip-up. This way when you pull it out of your pucket it is already perfectly positioned in your hand to be flipped open.
There is also something to be said for keeping the blade toward the back of the pocket, so the clip has to be on the right side if you are going to carry the knife tip-up. With tip-down it doesn't matter so much, but with tip up, if it is set up so that the blade opens up towards the front, (as in the back of your pocket is not holding the knife closed) it would be really easy to cut yourself if the knife accidently opened before you stuck your hand in your pocket. This is especially if the knife doesn't have a good detent.
Just my two cents. Really it doesn't matter though, however you are the most comfortable is the best way to carry your knife.