I think it is merely a matter of preference. However, I think having it unbuttoned looks best when you have a French cuff shirt. All of us here do things that reminds us of our 'sophistication' when they are no longer functional in our everyday life, like having a pocket square or a lapel pin. I just don't think that leaving two buttons open on jacket sleeves makes you less subtle, modest, or discreet. Just my opinion. "Look at that guy, leaving the top button of his jacket opened --- alright, so it's a Neapolitan suit. Why can't he respect the function of a button/buttonhole?" "Right, he should just get a two button suit or button them all up." "Pompous ass." That never happened, right?