Reddit mentions of Competitive Shooting: Techniques and Training for Rifle, Pistol, and Running Game Target Shooting

Both eyes.

My uncle was a firearms instructor and shot competitively. He taught me how to shoot after my father passed away. One day he noticed me squinting my left eye and said my father would have slapped me on the back of the head if he saw me doing that.

Shooting with one eye closed causes fatigue if you do it for very long. It will also cause you to lose visual acuity after a few seconds vs shooting with both eyes open. A.A. Yur'yev wrote a really good book with all sorts of awesome information like this in it. I like to reread it every few years or so as it seems I always pick up something new.

Here's a link if you're interested Competitive Shooting

Edit: Holy crap I never realized how expensive that book is.

Competitive Shooting by A.A. Yur'yev covers this in detail, but I have no idea where to recommend the book for cheap.

Apparently that book is worth a fortune now ಠ_ಠ