Reservations are particularly important for character experiences, for signature dining experiences, and for elaborately themed restaurants (such as the Sci-Fi Dine-In), but If there is ANY table service restaurant that you feel strongly that you would like to dine at, then I suggest that you make advance dining reservations for that meal.
At less crowded times of the year it would likely be possible to have some table service meals without a reservation. For example, Restaurant Marrakesh in the Morocco pavilion at Epcot can often absorb many unreserved guests. But during busy times of the year, and during the free dining weeks in the fall, all the table service restaurants are packed all the time. A walk-up might wait for more than an hour, or more often, not be able to be seated at all.
Any visitor to Disney World can make dining reservations 180 days in advance by calling Disney Dining at 407-WDW-DINE. If you are a guest staying at a Disney resort, you can make reservations for your entire trip (up to 10 days worth) beginning at the 180 day mark. This does give resort guests a bit of an advantage in securing choice reservations. Moms Panelist Kim does a great job of explaining the 180 + 10 day rule
HERE.
I hope you have a great trip ... and lots of great meals!