Having a knife in your pocket carries some risk regardless of tip up or tip down or bottom of pocket. Is gravity a friend with tip-down carry? Sure, but it's close cousin, momentum, doesn't care whether its tip-up or tip-down. A knife with a loose enough action for it to be jarred opened in a tip-up carry is just slightly less likely to also have it jarred open in a tip-down carry. If you are standing up, things are cool. If, however, you are sitting down at a table, bump the table (causing the tip-down blade to be jarred out), and you sit down, that tip might just be headed into your thigh. I've had this happen. And, the same action of bumping a table as you sit might just hit the blade post (or hole) in a tip-up carry and open that one as well. It might end up in your leg, stomach, or side. Where was gravity in these cases?
Gravity as a catch-all argument against tip-up carry is not valid simply because the effect of gravity can be easily lessened or even overcome by momentum or by any force that has a directional component opposing it. Gravity's effect isn't necessarily headed in the right direction. And, even if it is, if your body is falling, gravity is no longer helping that tip-down to stay closed. When we are standing on our feet, our mass causes an equal and opposite reaction to gravity - we press on the earth. These forces cancel out, or we'd either sink or start floating. Because our feet stop us from sinking, gravity continues to pull the tip-down carry, and the relationship of the components of the knife as they interact with gravity helps to keep it closed. If you're falling though, all parts of the knife are falling at the same speed. Gravity does not distinguish between the blade and the rest of the knife. This means that as far as the mechanics of the knife's components are concerned in relation to each other (ie, opening up), gravity has no effect. We don't spend our life's vertical, but in lots of orientations. I may have only finished my junior year towards a degree in engineering science & mechanics before switching to computer science, but I do remember a little about static forces, dynamic forces, and angular momentum.
Besides, when do the un-intentional openings occur? It isn't when we are standing up and not moving.

Yes, gravity can aid in tip-down carry in some cases. It can also work against you in ANY style carry. I'd much prefer to trust a well-made knife to prevent a serious cut than gravity. Gravity is not always a friend. Any knife with a reasonable amount of force keeping the blade closed will do so regardless of it's orientation, not withstanding the existence of a greater force of opposite direction. Any knife without a reasonable amount of force to keep the blade closed can open and cut you regardless of it's orientation.
The only truly safe way to carry a folder in a pocket is for the blade to be welded shut - and the weld better be good. Even a safety lock can get bumped.
There is no clear cut, single answer here. Just opinions and preferences.
Lesson over - knifeknut exiting podium.
I prefer tip-up. I also prefer that you use whichever you like!