Yes, if you have the money to do it now, you can get the tickets for today's price rather than after a few price increases before 2019! You can hold onto them as Robo said, since they can be used anytime. And if you are not sure how many days you want, you can buy a number you know will be your minimum(like 5 days), and can upgrade when it gets closer if needed, to make FP for all days, or at parks once you are there.
The only caveat is if a promo like free dining is offered, which requires you purchase a 2 day minimum ticket, then you would need to purchase those tickets with that package. But you could always buy those and save them for next trip!