Most people think that the only way to take out a Z is to nail it in the head. While that's a good rule of thumb, zombies are actually more vulnerable than just head shots. Sure, if I have a chance to crush a zombie skull I'm going to go for it, but sometimes, that's not a good option or you don't have the room for a full blown swing of your weapon.
Sometimes, you're caught with only your knife and then you might have some problems if thats the only thing between you and your new unlife.
Human skulls are hard, and although zombie skulls are decaying, there are other options.
Shoving your knife into the base of head right above the neck will immobilize the zombie quite nicely. The head may still be alive, and therefore dangerous, but at least its not moving anymore.
If you're in a situation where you only can reach the back, go for it. Anyway you can put a zombie down is a good way. Penetrating trauma to the spine is always preferable to blunt trauma. You only get one chance to screw it up.
The weaker parts of the skull include the eyes, a favorite target of John's, and the temple. Make sure your knife can handle punching through a skull. I like a nice drop point on a long blade, while John favors the tanto style with a shorter, thicker blade.