If you're booking a package, you only have to pay a $200 deposit. (Which, considering the cost of a Disney vacation, isn't much.) Payment in full is due 30 days before your check-in date. So I'd say sure, go ahead and book whenever you're ready to pay that $200 deposit. This way you have the room/resort you want all lined up. When/if discounts are released for your travel dates, you can absolutely call and make changes to your reservation - assuming there is availability at that resort. Having a non-discounted reservation doesn't automatically give you the discount, but if it's available when you call, they'll happily change your reservation for you.
If you don't get the discount you want/need and that means a Disney trip is off the table for you and your family, your reservation can be cancelled before the 30 day mark, and your deposit will be returned.
Like the previous poster said, you definitely are doing best if you book more than 180 days before your intended check-in date, so you can take advantage of the earliest chance to book dining reservations.