They'll unclip from pretty much any angle if you wrench them out hard enough... not that I've ever tried to pull them out in panic or anything...
It's a good idea to periodically check how tight the cleat bolts are, especially after first bolting them on. I checked mine yesterday, since they've only been on there for 400 miles or so, and they were decidedly looser than I liked, now corrected.
I've managed to loose a bolt on one occasion, although thankfully the cleat would still release. I've also had a cleat shift when uncliping, resulting in a "moment", not good.