I think there's big differences between head shots on squirrels and head shots on deer:

1. 30% of a deer's head is snout, and a hit there will sometimes result in a lostdeer. A squirrel shot in the nose or mouth with a .22 is almost always bagged. A lost squirrel is much more acceptable than a lost deer.

2. A deer shot in the head sometimes results in a disgusting mess, ie: eyes and teeth and brainshanging out. The deer then has to be transported, registered, hung and processed. This situation is disrespectful, and it sucks for other people to have to see it. If a head shot on a squirrel gets messy, you simply cut it off and move on.

I would almost never take a head shot on a deer. Even when finishing a deer off, I usually will shoot it through the ribs. Sometimes I carry a .22 pistol while deer hunting, and I will use that to finish one, doesn't make a mess and saves me half a dollar.