For Heathrow, CDA's are considered not only best practice, but are monitored and evaluated as well, being one of our Key Performance Indicators (KPI's) between the hours of 2300 and 0600 local (or 0700 for Easterly operations). If for any reason during a night shift, when the noise thing is the most "sensitive" to the locals, a CDA is not complied with, the details (operator, time etc) are recorded and past on to the powers that be, who I believe contact the operator if it resulted from the operation of the aircraft.