I'd suggest buying the chef a beer or sake if you want to do something (I've done that a number of times in the past). I've slipped a chef money for phenomenal service on a regular basis (I was going to a sushi bar in Sydney several times per week), but for the most part I don't think tipping is necessary. The chefs aren't expected to work on tips and receive a normal wage. It's really not much different from tipping or not tipping a chef who works in the kitchen.