Do my wife and I need to register at the local police station?

My wife and I will be travelling from Seoul, Korea to Siem Reap, Cambodia. We will be transiting in Guangzhou. Our layover is about 21 hours and we are not going to be getting a hotel. We are planning on meeting with some friends and then heading back to the airport. I have heard that registering with the police is not necessary for stays of less than 24 hours.