This may be an old topic for some, but newer members may be unaware of this fact. In that I am older than this topic, and most everything else for that matter, I will comment without reservation.
I think tip up or tip down carry is a matter of choice, as many others have stated. I will add however, if you carry a knife for last ditch self-defense purposes, it is important to train, train, train with whatever knife you carry. I believe you should make a reasoned choice between tip-up or tip-down, hole, disk, stud, whatever, and stick with that choice with all knives you carry.
Similarly, with regard to automatic pistols, I always cautioned against carrying a 1911 on the hip and an S&W 669 in an ankle holster. Under stress it is important you do not have to think about the manual of arms associated with the weapon you have accessed. It is important you react quickly and instinctively.
I think the same is true of knives.