Seems like this thread has gone in two directions:
1. Zipties - good or bad?
2. I was bitten by a tip-up/tip-down knife
My opinions:
1. I'll try out the ziptie on one of my tip-ups. It's a lockback, so I don't know how the closing bias will affect the opening. I think if the ziptie didn't get a good grip on the back of my pocket, the knife would close (a potentially dangerous situation). But of course my curiosity means that I'll definitely try it out
2. I once sat down with a tip-down in my back left pocket, my rear-end caught the thumbstud, and the blade opened, poking a hole in my pants.

Good thing it was a small hole. As a matter of fact, I'm wearing that pair of pants right now. So I wasn't bitten, but my pants were.
I actually think that tip-down is more dangerous, since I carry tip-up knives at the back of the pocket, as some people have mentioned. Whenever I put my hands in my pockets, I run my finger down the top of the spine of tip-down folders, to make sure they're closed.