Registration for branches

We are a company in ITES segment. We offer our services to all over India. We have branch offices in 2 locations only. All our billing are done from our Registered office in Chennai. We take some services for all our works in customer place and we get all their bills addressed to our Registered office. Only our branch rent and phone bills and some utility bills will be in the branch address. I would like to know in this circumstances whether we need to register our branch offices? most of the states where we render services we do not have branch offices. Every thing is done from our Registered office. Can anyone give me a clear picture of why I should register each of the 2 branches?