It depends on the cake design. Our cakes are served on a cake tray. Usually we will write the message on the tray below the cake. For sculpted cakes, we prefer not to write the message on the cake itself with buttercream because it could get sloppy if not on a flat surface of the cake. Round cakes usually can fit a small message ontop of the cake but they are also served on a tray that a message can be added to.