If you can size up, there's no reason not to, but I've found that, particularly with some of the Italian brands, you run out of runway pretty quickly when they size super small. I wear an L in most t-shirts (though an M in some absurdly 'Merican-sized brands) and the largest sizes of some Euro jerseys are too sausage-casing-esque to fit me, even if they're labeled 3XL.