Simpson's Forensic Medicine is a textbook that has some relevant information. TW for some very graphic pictures. Not gonna link but you can find free copies online.
There's a section on how to recognize shotgun wounds after the fact. There aren't any statistics on real world instances but it discusses how location affects impact. It seems like its really not a concern of where on the skull you place the muzzle so much as it is ensuring that once you've placed it there, it stays put. Like if you get a shotgun shell anywhere into your skull, it's gonna explode. Very close to guaranteed. But if you try to aim for the temple and its a little out of reach and you slip, then it might graze your skull and not kill you.
So I don't think it's a one size fits all answer, I think it depends on the shotgun and your arms. Whatever you need to do to be sure that when you pull the trigger, the muzzle isn't sliding a centimeter to the side is fine. I read a story about a guy who built a rig with a pully system for his shotgun cause it was longer than his arms. Tied a string to it and everything, You have some flexibility is my point.
Also in regard to the book, the photos in there are pretty much the exact worst thing you'd imagine they are. Definitely all successes though.
