Doing hundreds of crunches isn’t the most effective way to strengthen your core or to get a ripped six-pack – not for climbing, and not in general.
As I mentioned in an earlier Lifts I Love entry, most climbers (and probably most people in general who work out) focus too much attention on the abdominal region and not enough attention on strengthening their lower back concurrently. That being said, it is a good idea to develop and maintain strong abdominal muscles, not just for climbing but for life in general. Balancing opposing muscle groups is always a sound plan for a healthy and functional body.
My favorite abdominal exercise these days is medicine ball leg lifts, performed with the medicine ball clasped between the feet. Check out Medicine Ball Leg Raises – Abs Exercise Guide with Photos for detailed instructions on how to perform this exercise. I generally do not include the switching of the ball to my hands and overhead, as I work these upper body muscles in other exercises using similar motions. Whether you do this portion of the lift is up to you, taking the rest of your lifting program into consideration. Be careful not to drop the ball on your face!
There are lots of other effective abdominal and core-strengthening exercises out there besides this one, including certain yoga poses, in addition to more standard crunches and hanging abs exercises (using a bar and doing knee lifts, for example). I suggest that whatever abdominal strengthening work you decide to include in your training program, perform the exercises at a level of intensity that isn’t aimed at endurance (i.e. hundreds of repetitions), but at a more strength-training level of repetitions, so that you’re working hard to do 10 or fewer reps per set. Complement every ab-focused exercise that you do with a lower-back (opposing-muscle) exercise. Strive for balance in your core-strengthening efforts.