It really depends what time of year you're going and how many in your party. Most places (save the very popular) would be able to accommodate a party of two at off-peak dining times (early or late dinner, for example). During slower months (Feb, May, Sep, Nov) you also shouldn't have problems at some of the less popular places.
As the PP stated, the resort restaurants are generally easier to get into (with the exception of places like Chef Mickeys, 'Ohana and other popular/character meals).