I close one eye when focusing. For me, either eye works, makes no difference.
Even the auto focus on a camera can get fooled. In business, especially with each group photograph, I had to make sure the camera was focusing on the faces of the people and not some other object. With my Canon SLR’s, a double check was necessary to make sure what was supposed to be in focus was in focus but when pressing the shutter button part way down it would re-focus, a real pita. It got to the point that I turned off the auto focus as it would be another item to mentally check off, to make sure what was supposed to be in focus was in focus.
I had more than enough to think about and having a beautiful photograph out of focus was a big no no for me.
I make photographs as a return ticket to a moment otherwise gone.