Jet Blue's reward redemption levels are based on the published base fare of the flights you select at the time of booking. So the number of points needed will follow the fares.

If you are booking for a high demand time period (Christmas-New Year's holiday, for example), the fares are not likely to go down from where they start out, and booking immediately would be desirable. For lower demand time periods, I agree that waiting for a fare sale is typically a good strategy. Since Jet Blue does not have black-out dates and the reward redemption levels follow the fares, this works equally as well for award travel.