It depends on the size of your hands, the length of your folder and where you carry your folder.
In the front, tip down is easier, it's more natural for your hand to draw and open from tip down.
In the back, tip up is easier and more natural, except when your hand is too small or the folder is too long for you to reach the hole with your thumb. Then you'll have to change grip on the handle to reach the hole which may not be desirable. Or else, you'll have to wrist-flick the blade open if you want to open it without changing grip.
Tip down in the front doesn't have this problem. You can wear a long folder, and your thumb will be able to reach the hole because the hole is near to the pivot end. But the long folder will be uncomfy you when you sit down.
I have about 15 spydercos and only one of them has opened in the pocket. That was an old native with a purpose-made very "soft" lock. I don't carry it.
None of the FRN and SS enduras and delicas I have has ever opened in the pocket. The Military's ball detent is also quite secure. The gunting is best carried in a holster unless you screw the pivot really tight.
Linerlocks, unless they have a really good detent, shouldn't be carried tip up unless the spine of the blade presses against the seam of the pocket. I like to carry linerlocks tip down. The millie(new one) is a very good linerlock, IMO.
Another brand that has positive closing pressure like a lockback is the axis lock from BM. I like the axis AFCK, which was originally a spydie design.
Lastly, the type of trousers you wear often determines your carry style, if you are thinking self-defence, it's a good idea to wear the same way all the time. I usually carry one 4" for emergency purposes, always the same way, then carry two or three others for utility in all sorts of ways, changing them all the time, tip up, tip down, clipless, whatever.
It's your choice in the end.