Hi Jennifer,
Yes, it is definitely recommended to make your reservations well ahead of time if you are able to (for example, you have booked your package and know the dates down the road when you will be staying). Many of the popular dining experiences do fill up 6 months ahead of time, one example being
Cinderella's Royal Table. Breakfast and lunch (being more cost efficient) seem to fill up quicker than the dinners, but dinner reservations are a hard find too, closer to your date of arrival.
One tip I can give you is not to fret, I was vacationing once and was not able to get a reservation at many of the places I wanted to go, no matter how many times I called, but when I was in my room at the resort I tried to call (just for the sake of trying!) and wouldn't you know it, I was lucky enough to get most of the one's I wanted due to some cancelations from other guests. Moral of the story,
just Keep trying, just keep trying!!!!!
Best of luck,
Brigitta