Amsnag does a good job of showing which travel time is best but it still can't show you which purchase time is best. At one time there was a benefit to buying as early as possible but that hasn't be the case for a while now. If there is a new trend as to when discounts are likely to arrive for a given route I don't think anyone has deciphered it yet.
In theory, it should be do-able to keep track of what amsnag reports for current fares for each day (up to 12 months into the future?) for each route, then analyze how those fares change as the actual travel day approaches. I've seen some websites that do such an analysis for air fares, giving suggestions (which I believe are somewhat better than simple "guesses", since they are based on past experience) on whether fares are likely to go up or down over the next several weeks/months.