Tipping is a tricky issue in China. It's officially forbidden by the government, and locals simply don't do it. In general, follow their lead without qualms. Nevertheless, the practice is beginning to catch on, especially among tour guides, who often expect Y10 a day. Official CTS representatives aren't allowed to accept tips, but you can give them candy, T-shirts, and other small gifts. You don't need to tip in restaurants or in taxis.
In Hong Kong, hotels and major restaurants usually add a 10% service charge; this money rarely goes to waiters and waitresses. Add on up to 10% more for good service. Tipping restroom attendants is common, but it is generally not the custom to leave an additional tip in taxis and hair salons.