Visa for china if getting a private tour and not packages

Hi,
I want to inquire if i need a letter in going to a china if we only get a private tour and not a package one. I am confused because some of my friends told me I need to secure an invitation letter from an authorized travel agency and some said i don't need it since it will be just for a tour? please enlighten me on this. If i hire a private tour guide will i need his/her invitation?