We've debated this in some fairly robust terms over the years! Tipping isn't a part of Chinese culture at all, but as China becomes more cosmopolitan the practice is, be it for better or for worse, creeping in. It's still very limited though.

What's normal is simply not to tip at all. The only people who really tip in China are those who come from a culture where tipping is pretty much obligatory and who believe that their own system is best and for export to the world (i.e. Americans) and/or those who come from a culture where tipping is normal practice and without giving much thought to the matter just carry on as normal when they are in China (mainly Americans, but quite a few others also).