Whether you shoot with one eye, or with both eyes open is going to depend on whether or not you are shooting with your master eye. If you are right handed and have a right master eye, it is best to learn to shoot with both eyes open. If you have a cross dominance, such as right handed with a left master eye, you must adjust by either learning to shoot left handed (so the master eye sees the sights) or shoot right handed and close the left eye (so the master eye does not see the sights and confuse the sight picture).