I don't know about anyone else, but when I look before holstering, I'm actually looking. It's not just a movement. It's knowing that if that critical moment comes, my head will actually be in the direction of any other possible threat.
I think it would make more sense to keep looking at the target you are shooting at until you are sure the target is no longer a threat. You will feel a bit of a prat if the target you have just put down shoots you when you are busy looking around you. That's of course you survive.
PS Who are these people setting themselves up as experts on what way you should react in a fire fight. I take it they are all experts who have being in numerous fire fights and have years of experience on how to react in dangerous situations. Or maybe they have read a few books ans decided they are experts.