Being a balisong artist, I prefer to carry tang-forward.
Now, for conventional folders, I carry tip down, the way God intended. It says so right in the Bible, "Let your conventional folding knives be carried tip-down, for anything else is from the evil one." I may be paraphrasing a bit there, but it says something like that.
A friend gave me a tip-up knife to take home and sharpen for him just a few weeks ago. I forgot that that thing was there and reached into my pocket and almost sliced my hand open. The blade had fallen open just a bit. Fortunately, I felt it before I got cut (fortunately, his knife was rather dull).
I have a good friend who is a mechanical engineer. One of his basic rules is, "Don't fight gravity. You can't win."
Tip-down is intrinsically safe; it uses gravity to keep the knife safe. Tip-up is obviously dangerous; it fights gravity... and that's fight that you can't win.
Maybe when I'm rich and not famous, I'll buy myself a week on the International Space Station and carry tip up. Until then, while I'm subject to gravity here on earth, I'll carry tip down.