I prefer tip up carry. Like it has been said before, when you pull the knife out of your pocket, your hand is naturally in a position to deploy the blade. When I pull the knife out of my pocket, I insert my thumb into the pocket itself while my fingers remain outside around the pocket clip. Once the knife is slid out of the pocket I can open it immediately, as my thumb has been positioned over the thumbstud/spyderhole simply by the action of pulling the knife out.
If the knife is tip down, you have to rotate the knife 90 degrees backwards into your palm before you have a surface to oppose the force deploying the knife.
I've heard arguments that tip down is more secure than tip up for blade retention. However, if you carry a tip up knife as far back as possible against the corner of your pocket, then the knife can't open in your pocket.