If a person is liable to pay service tax, then he is required to take service tax registration. In my view, in your case, if any service liability is there, then service tax registration is required.
Further, if any benefit in respect of the export of services is proposed to be taken, then you should get service tax registration.
Presuming that your rent income has not exceeded Rs.10 lakhs in the preceeding financial year i.e., 2012-13, you are eligible for exemption this year and no service tax registration is required. In my view, no registration is required simply because because your export turnover is more than 10 lakhs.