Depends on the time of year you are traveling, but usually the best price is early. We purchased early the last time & a sale came up, but since we hadn't picked our actual tickets up yet, we were able to rebook at the new sale price, so unless something has changed (I would check their current policies), I don't think there's any risk to booking early.
If you're thinking of booking a sleeping room, those are definitely less expensive further out.
Best of luck with your trip!