Agreed... anything you really want to do, book now. (I'm taking an average of 7 to 10 reservations per week now.) With such a short window of opportunity for businesses here, things fill up quickly.
Also note that the booking kiosks on the pier (as seen in the photo of Juneau above) are often a big waste of time and money. You'll pay the same or more than booking on the cruise line, you won't get the security of a ship's tour, and you'll probably end up waiting around for hours while they "put the tour together"... with shorter and shorter port times, I'd hate to be standing around waiting anywhere...
Better not for now... signatures get me banned!