When in Japan, don't tip, especially in restaurants. The service charge is built into the price of the menu item.
If you do make the mistake (as I've done), of leaving a tip for a waiter/waitress, you are actually creating a great inconvenience for the staff, since an employee must stop whatever they are doing, and attempt to chase you down, for the sole purpose of returning your money. If the pursuer is unsuccessful in finding you, then you've also created a situation in which the employee has now failed in their task, which, over here, is a big deal.
Exception to the rule: If you are invited to dine at the New Sanno Hotel (Tokyo), which is run by the U.S. military, for military personnel and retirees, charges U.S. prices, and accepts U.S. currency, then tip accordingly.