I used to carry tip-down for years. Even developed a very fast draw-stroke for S.D. purposes. But every time I'd thumb the knife open, it ended up in the wrong position. (Edge up in my hand).
After some experimenting, and a post someone made on B.F., I realized my draw-stroke would actually work better on a tip-up knife. As long as the handle of the knife wasn't too long, I could get a secure grip on the knife. As for the positioning problem, I solved that by using Waved folders. The draw-stroke is even faster, and the knife opens in the right position, every time.
Generally though, if the folder has a long handle; tip-down is still the way to go.
After some experimenting, and a post someone made on B.F., I realized my draw-stroke would actually work better on a tip-up knife. As long as the handle of the knife wasn't too long, I could get a secure grip on the knife. As for the positioning problem, I solved that by using Waved folders. The draw-stroke is even faster, and the knife opens in the right position, every time.
Generally though, if the folder has a long handle; tip-down is still the way to go.