Well, the first thought that comes to mind is that you should book your room early if there's a specific room (or room type) that you must have. The standard FlyerTalk advice about playing the upgrade game: If you really must have a suite (or a first class seat), then that's what you should book.

The next thought that comes to mind regarding hotel booking is that there are certain known periods in Japan that will always be extremely busy (and probably very pricey too): Golden Week (starting at the end of April and going into the first week of May), the New Year's holiday, the o-Bon holidays (mid-August), etc. Other specific days might also be busy at hotels: Valentine's Day and Christmas Eve, come to mind, along with any 3-day weekends. Naturally, local events can have an impact as well: cherry blossom time (March or April?) as well as fall foliage viewing (November-ish?) can be busy times. Of course, if some big festival or conference is in town, all bets are off!

Having said all that, it seems like you have plenty of time to make plans. The prices this week may have nothing to do with next week -- let alone next year. Why don't you monitor the rates for a while and see how they fluctuate?

Also, do you have status with Hyatt or other chain? That won't help with the price, but can help you get a better room.

Enjoy your planning!