Descending into Boston on my first commercial flight into the United States I opted to follow the guidelines shown in the FAA AIM. Having received a "descend when ready clearance" I made the obligatory leaving call and was promptly advised in the most Bostontonian way that I could shove standard procedure and to stop clogging up the frequency.
I find that when in doubt, take it right back to basics. In this case, are we radar identified? If yes, then only make the call if you have reason to suspect based on your mental picture of the traffic situation that ATC has forgotten they gave you a "descend when ready" clearance. If you are not radar identified, then make the call. I take this stance because controllers can be working multiple frequencies, most of which I may not be aware of. If the controller has told me I'm identified, then I'm a part of their scan, so I don't want to interrupt them will a needless call unless I suspect or know their impression of the traffic situation is incorrect.