An old tailor once told me that shirts has to be tailor made for a crease to hang nicely. Even then, the crease shold not go all the way to the seam at the shoulder, and it should run on the back side of the arm. That means you can't flat iron the sleeve. A proper crease is thus harder to attain than a creaseless shirt. For manufacturing convenience, confection shirts are made with a cut not suitable for creased sleeves. To my eyes, (and the old tailors) flat ironed creased sleeves looks cheap. Especially if the collar is creased as well. And if done right,you don't need a sleeve Board to get creaseless shirts.