Carry my SOG Twitch II tip up, when I pull it out of my pocket, my index finger is already on the "opener", and with the assisted opening mechanism, I can have it open as soon as it clears my pocket.
I also wear my keys on a belt clip and normally don't carry anything else in that front pocket except a pack of Camels sometimes... (New Years resolution, pocket soon to be empty)
Tip up with my EDC.....a Kershaw Blur.........I can remove the knife from my pocket and deploy it instantaneously. It's more natural and I don't have to change my grip.
I carry my Spyerdco Tenacious tip down just because it came this way from the factory.You can change it to tip up but after over a year carrying like that im used to tip down.
It depends on the knife, the best knives will allow you to carry either tip up or down.