I replied already but I want to add that we have run into a hurricane, sickness and injury on trips that kept us out of the parks on days we planned to be there.
Last trip, two members of my group had to go to the hospital in two separate incidents. Plus we had a hurricane. Another year, my son won a snake-in-the-can at the resort arcade and accidentally shot the snake into my husband's eye. Scratched cornea and a lot of stress, we skipped a day of park going.
Anyhow, unless I get a good deal, I always hold back a day or two because it is easy to add days, but if you don't use them, you lose them.