My son did X-ray only when we ted our I485 applications. Last week the RFE we received for his case says that TB test is a must-have item (even though his x-ray shows everything is ok). So we took him to the doctor to do TB test again. My son's case was approved only 5 days after CIS received the sealed evenlop (from the doctor).
If you receive a RFE for TB-test, most likely your case will be approved very soon after you mail it to CIS.