Ultimately it is preference, but there are a few things which come into consideration. *only considering pocket carrying here*
(this is all just my opinion)
1) Hand size - If your hand is on the smaller side or you're just not too dexterous, a tip up carry might be awkward for you to deploy the blade (quickly) while pulling it from your pocket. You may find you establish a grip on the knife to pull it from the pocket and then establishing a new grip which puts the knife in a better position in your hand for which to deploy the blade.
2) Knife size - If the knife is rather large, it can cause the same problem as above.
3) Clip positioning - If the knife rides extremely low in the pocket - you could yet again face that same problem mentioned above =P
4) Blade retention - If the blade retention is not that strong and you are able to simply flick your wrist (no thumb action required) - this can overcome the aforementioned problem because you won't need to switch grips to maneuver your thumb to the deployment stud / hole. Then again if the retention is so little that the blade may open on its own while in your pocket, you run the risk of poking yourself when going into your pocket for the knife due to the tip being up (happened to me with my Gerber EAB... I poked my index finger, said ow, and wondering what just stabbed me, shoved my hand back in there and poked my finger again... I'm a smart one

)
5) Position - Depending on whether you have the knife clipped to your front or rear pocket, a tip up or tip down may lead to a more natural gripping for you. Personally, the front pocket feels better with tip down while the back pocket feels better with tip up.
6) Flipper - I don't personally own a flipper (yet), but I think a blade with an extended tang flipper deployment may accidentally deploy pulling out your knife when it is carried in tip down (like a reverse wave feature)... Then again, this could be what you are looking for
Maybe there's more, maybe this is all just drivel - but this is what I've thought about when considering how I wanted my knives to be carried =P