Basically here are the new ticket rules:
If you buy a ticket today, you will pay today's prices. Then it will be good until December 2018. Until then that ticket can be used without needing to pay any increase. After December 2018, the ticket VALUE will remain, which you can use to buy a new ticket; that new ticket could be less expensive than the original one (i.e. If the original was a 10-day park hopper and you instead bought a 4-day base) in which you wouldn't have to pay anything out of pocket and the remaining balance would stay in your account. Most likely, however,
ticket prices will have increased, so you will pay the difference between your account balance and new ticket price. Note: no one has first hand knowledge of the exact mechanisms of how this will work as no tickets have expired yet.
So, if you have the money now and know for sure you will be going to the parks, it doesn't hurt to buy a ticket now. Ticket prices increase without warning, so if you wait you may pay a higher price.
But in terms of pre-booking the only time when you NEED to have a ticket in your account is for fastpasses (so 60 or 30 days before check in). You can make resort reservations, dining reservations, tours/special events, etc, all without having a ticket assigned to your account.