I second what others have said.
|- is the traditional way of inserting shirt sleeves and other wider/less formal sleeves.
|\ is for tailored and more formal garments, but it's the classic way for more fitted sleeves. It also means that the final seam goes around the arm rather than under the arm and so is generally reckoned to be more comfortable.
ETA - on the whole it depends on the shape of the sleeve cap which method is more 'correct'. A flattish sleeve cap (most shirts) is best done by the flat |- method. More shaped sleeve caps are easier to work by setting in, |\.