Registration at local police station

My question is about registration while living in Guangzhou: I understand that when holding a multiple-entry visa you have to go and re-register your address every time you enter China. But what about residence permits? If I have a residence permit valid for a year, and my registration paper says my address has been registered until the date my residence permit expires, do I still have to go and re-register if I f.ex. visit Hong Kong and come back? Or do I only have to re-register when I get a new residence permit? No-one has been able to give me a definite answer.