Book early. The market has moved decisively toward early bird pricing, and the deep last minute discounting that used to reward patience has largely disappeared. Waiting now tends to mean a higher price and a thinner choice of cabins and departures, which is the opposite of what people expect.
Booking early also gets you the cabin you want rather than what's left, and on a ship where bow cabins are the last to go, that's worth something concrete.
One thing to be clear about: if the price drops after you've booked, you won't get the difference refunded. That's the standard across operators. Upgrades do sometimes happen when a ship is reshuffling its manifest, but treat that as a bonus rather than a plan.