At a non-wounded animal, I would be hard pressed to take a head shot. I have taken down one deer with its ears shot, and another with its jaw blown off.
If an animal is wounded, any shot that will help take it down and be recovered is ethical.
As for buckshot, I quit using it before I was old enough to drive. I shot at a single deer with it, not a single pellet hit at 30 yards. I recovered or helped track several other deer wounded with it, they traveled long distances. If it doesn't have feathers you should know exactly where your bullet is going, not spray 12 small under-powered bullets at it and hope they go where they are supposed to. I won't go as far as to say it's not ethical, but I don't use it and won't take anyone out with me who does.