Booking early at rack rate guarantees you the room - at rack rate. If you are counting on a discount, I would wait so that there is additional inventory to incentivize a discount. They will only "apply" the discount if the room is available to be booked with a discount as a new reservation. The consensus is to book what you are comfortable paying at rack rate, if there is no discount or a small discount. Then, when/if AP discounts are released you can see what resorts and rooms are available and what the discount level is and then change your reservation to something more favorable since already having that room booked does not give you any priority to a discount.
Of course, no one knows how SWGE will affect onsite bookings, but AP Discounts for May 2019 were plentiful, generally a great time for discounts, especially in the early through mid-month part.