I would like to add my 2cents to this one. The "monitoring CTAF" call is irritating to ATC as technically we are still required to pass traffic etc after this call. From an ATC perpective we couldn't care less what you are monitoring and it creates doubt that you have "changed to the CTAF" when in fact you still want a traffic service. If you want to continue to recieve traffic, say nothing and respond when called. If you don't want to be hassled when in a workload intensive part of your flight, report changing.
I would like to add that when aircraft are operating into destinations where the is no VHF on the ground the problem of aircraft not reporting changing to the CTAF is compounded and creates high workload and frequency congestion of flightwatch HF. For example, ABC is inbound to YXXX estimating there at 30. They didn't report changing to the CTAF. At time 28 DEF taxys YXXX on HF and advises they have copied ABC. Flightwatch relays this to ATC ,they tell flightwatch no additional traffic and then attempt to call ABC on VHF. No response, probably out of range. ATC call FW and ask them to pass traffic DEF to ABC. FW is mad busy with all the calls going on, they have 4 aircraft standing by to give taxy or arrival reports, and try a few times to call ABC. No response from ABC. FW call ATC and advise them no contact with ABC traffic not passed. The whole process probably wasted 2-3 mins of time which could have been avoided if ABC reported changing to the CTAF.