Tom, :wavey
Do you drive your convertible daily/often in Boston and is it garaged the majority of the time? If so I would throughly wash monthly and coat with protectant of choice (RaggTop in my case) every 2-3 months for maximum preservation of a fabric top. When the protectant stops beading like the protected paint, it is time to renew.
If you are like me and only drive your convertibles for pleasure excursions and garaged all other times, once a year is sufficient with vacuuming and rinsing only needed. DavidB's newsletter had a great article on top care which included the thread care and its requirement for consistent care and protection. Excellent read for all convertible owners.
Lived in the Boston area for 43 years and the pollution (fallout) is terrible. With the Big Dig there is even more particulate matter consistently a float.
Just got back from Boston and Newport RI this week and I had to rinse the black car (LS400) everyday because of pollen and fallout!
You probably already know this but clarification of conditions in which one uses one's convertible (canvas) greatly effects frequency of top care.
Try a quality WW as a canvas top drying towel. They work better than a regular MF.:xyxthumbs