My fiance used to have a white mustang convertible with a white canvas top, and I couldn't imagine not washing it. After about six months that thing would look a nice shade of brown.
The Porsche brand products are supposed to be very good, as well as the Raggtop brand products.
My mother recently bought a 330cic and I only wash the top every once in a while, but vacuum it every time. This seems to keep it nice and clean.