I have some bottom of the line, cheap m520's. I run them as loose as they can go, and I think I've only accidentally unclipped once or twice. On the other hand, I've come out of them pretty much every time I've fallen, including during one of those 0mph falls. Occasionally, my top foot will get stuck if I fall when I'm going slowly, but that's not that big a deal, other than it's sometimes awkward to get out. The foot in the direction you're falling will almost always come out, since you'll naturally turn your heel outward to try to stop your fall. And I've yet to high-speed crash and not unclip. I wouldn't be too concerned, and if it's really an issue, Shimano also makes cleats that release from any direction (though I did borrow my friend's shoes once, and I definitely DID accidentally unclip from them...multiple times on a short ride).