Mostly, it is recommended a regular cleanliness of whole AC system. But this practice become more significant when you have children or any allergic patient in your family.
Second mostly recommended time to clean AC system is after winter, when the system is turned off for long duration. After any work related to construction and renovation, AC system must be cleaned thoroughly.
It is also recommended to clean the system when you move to a new residential unit either it is new or old because you do not know the exact history whether it is cleaned by any professional team or not.