I feel that if the knife is small enough that your thumb reaches the stud, hole, or disc when it is drawn from your pocket tip-up, this is how it should be carried. This way you can draw the knife in the same grip that you will open it. For me, however, this only works on knives with blades under about 3.25" or so. Larger knives require me to adjust my grip before opening, so I prefer tip-down where merely have to flip the knife 180 degrees into my palm instead of "scooting" my grip up the handle to get the right grip for opening.
There is also a very special thing that can be done with tip-down blade-hole knives: the "Spyder-drop" as it used to be called in ads. This is where the knife is withdrawn from the pocket by holding the blade hole and the handle is flipped open with a quick twitch of the wrist. The blade may be used from this grip or "hopped" quickly to a more usual one. In my experience this is the fastest deployment method among practiced users, though I have never met someone proficient with Emerson's "Wave" system.
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-Drew Gleason
Little Bear Knives
[This message has been edited by Corduroy (edited 06 August 1999).]