I have hunted all of my life. I have killed numerous critters for food and some for varmit control. Head shots are indeed a quick way to dispatch something, but not always the neatest. Take squirrels for instance, I have killed and ate hundreds in my life. Head shoot a squirrel and they flop around fairly regularly (nerves) for a few seconds. I shoot them in the boiler room (heart) and not so much as a twitch, just dead. Birds react much like that as well. I killed a deer once that was heart shot with a 308 and ran 200+ yds before piling up. Upon cleaning the deer, I found that it had ran all that distance with a heart that was cut into 3 seperate pieces, so there will always be exceptions to just about anything.
But with that said, I am a firm believer that bullet placement is of the highest priority to a hunter, or at least should be. If you hunt elk or larger game with a .223 or any thing smaller than a 30 cal., you really have no place in the woods and are doing nothing but giving hunters a bad rep. Bring enough gun, or pick a quarry that is within the range of your choice of weapon. I use a 25/06 for all my deer hunting and my wife has an awesome little 257 Roberts that is an accurate of a rifle as I have ever shot.
Peace
Asmallvoice