For the most part there is no tipping in Japan; leave change behind on a restaurant table or at the cash register and you may find yourself chased down the road by staff desperate to return your "lost property".
However, tipping is customary in a few instances. A tip properly given at these times will impress the recipient with your cultural awareness. Tipping is common when staying at a ryokan, (Japanese-style inn), in which meals are served in your room. The tip should be handed directly to the server or maid. 1,000 yen in a small envelope is customary (up to 3,000 when staying consecutive nights), although a small gift from one's country or another location in Japan is also acceptable.
You can try to let a taxi driver "keep the change" but you may find that the driver at first refuses the tip, and tries to hand it back to you; it may take a couple of goes to get him/her to take it! If you hire a driver for the day then a tip of 500-1,000 yen is customary.