Many of us can shoot 1" groups at 100 yards...on the range, but in the field, such placement doesn't always occur so well. I once had a perfect frontal head shot on a hog messed up by a slight movement on the hog's part, moving its head to the side just as I fired. In approximately 1/8 of a second, the hog moved its head about 2", enough for the bullet to wizz by, clip the ear, and plow through the shoulder and down and out of the belly. The hog died just fine, but the point of impact was way off from where I intended to hit.
Head shots are great. I prefer a quartered away shot where the shot enters the neck near the head and crosses into the skull. Here you get the benefit of a CNS shot disrupting the spine, brain stem, and/or brain.
The problem with head shots is that hog heads tend to move about quite a bit. I like to watch a hog to seem if it moves in a patterned behavior before taking the shot. For example, a hog may put its head down for 3 or 4 seconds to eat and then lift to check. I wait until after he checks and the fire when he drops it down to eat again. Or, better, I like to shoot when hogs put their heads up to do their security checks. They will often stand still, hold their head still and sniff and listen. This is when they are a perfect target.
I am not a fan of just neck shots and am not a fan of broadside behind the ear shots. Both types of shot have the potential to miss vital structures and if the bullet doesn't pass close enough to the spine or brain to get the benefit of direct damage or hydrostatic shock, you will be chasing the hog into the brush. I have watched at least a couple vids on Youtube showing "behind the ear" shots that did not drop pigs and have recovered one pig with a healed "behind the ear" shot.
The problem with "behind the ear" is that the ear really is at the rear of the head and so you can end up shooting through neck muscle and nothing more without hitting the head or spine.
I have seen a lot of folks shoot "behind the ear" and they end up shooting behind and well below the ear. This is a good shot that is apt to hit the spine near the junction of the skull.
If you don't hit and significantly disrupt the CNS, expect the hog to run. Hogs can run over 100 yards with a blown heart and in my experience, nearly 200 yards with a lung shot (though most fall within 100). Many shoulder shots you see that drop hogs in their tracks are going to be hogs that also had their spine disrupted, either directly or via hydrostatic shock. Don't count on a shoulder shot necessarily dropping a hog.