chest x-rays for medicals are normally only conducted on applicants from high-risk TB areas and are not the norm. for example the western province in south africa (highest TB rate in the world) as well as some areas of india. although i know people who stay in these areas and have recently been for medicals without having to have an x-ray done.
medicals have to be re-done every 3 years prior to the renewal of your residency status - with the notible exception of people who work in the food, hotel and medical profession where a yearly medical is required.