Depends on the lock type & opening device.
If a liner lock/frame lock, I prefer tip up, so it does not open in my pocket accidentally.
If an axis lock or a lock back, it depends on whether it has a stud or a hole: stud=tip up, hole=tip down.
I guess when it comes down to it:
I like the tip down, with a thumb hole, so I can just pinch the blade by the hole, draw the knife from my pocket & flick my wrist down, to drop the handle into my palm, all in one motion.