That's the kind of thing that doctors need to monitor, and since yours is aware of this potential, and your medical history/status, s/he may be watching out for your safety. It might not be medically prudent/responsible NOT to see you monthly to make sure everything's going alright.

It may depend on the doctor, clinic, state law, or bureaucratic intervention (instructing a particular doctor to do so). I know some patients who are required to see their doctors physically every month, and others who just have to physically go in and pick up their scrips (which does not incur an office visit). Coincidentally, the one that has to see the doctor is also on Fentanyl, but I've been told it's some kind of bureaucratic requirement along with monthly drug testing.

Doc