Certain airfields require CDAs. When controllers start giving you radar headings and you know a CDA is required on the landing booklet, then it is obvious. At other airfields where CDAs are not required, I still try to do them to stop the cabin pitching up all the time- for comfort the passengers will just feel a steady descent. In the US where idle power descents are required, you just obey their requirements. Nobody instructs you, it is assumed you know what you should be doing.