The jaws invariably splay when you apply force, even if you're using them correctly. I've come to the conclusion that it's just intrinsic to having a moving jaw that moves easily enough to be useful.
At this point, if I expect to need the wrench ever again (or need to reuse the fastener), I'll just buy a proper wrench for it. The couple of Crescent wrenches I keep are for emergencies and plumbing (because I don't have combo wrenches that big and do it infrequently), nothing more.
Vise-grips? Another tool that I find is more apt to round something than grip it properly. If I'm reaching for them, I figure I'm already so dicked that I'm unlikely to make it worse. Once in a while they get me out of an otherwise unavoidable trip to the hardware store.