Reuben Sandwiches are traditionally served hot to take advantage of the freshly butter toasted, crispy bread and melty Swiss cheese.  Reubens can also be served at room temperature but they lose some of the ooey gooey cheese appeal, and warm toasted goodness.  I do not recommend serving them cold because they taste like an entirely different sandwich.  The flavors are not as pronounced, the bread is no longer crispy, and the melted cheese is no longer soft and melty.