To take it one further, it would be nice to have a revision schedule. If I'm going to take a particular route in six months, and I know there's no planned revision within the next six months, I might as well buy it now, and then update the addenda just before leaving. But if there's a revision in progress, and it should be out in four months, I should hang tight. And if I'm putting together something extended that combines several routes, I could do some and some.
They suggest waiting until close to your trip, but if a map has been recently revised and isn't about to be revised before the trip, there's no benefit to waiting, and you can start planning in more detail with the map(s) in hand.