I didn't need any of them during a recent trip. It's only necessary if you have a problem and need to prove you had a reservation. Of course as soon as you don't have them is when you will need them.
i actually almost never take them with me and don't have problems. if you are worried though, go to concerige at your hotel (if you're staying on properity) or guest services at the park and they'll print them up for you to have.
If you have a pre-opening character meal at a park, you'll need that number most of the time. I have to make two copies, one to take to the park and one to keep in the room.But I ain't compulsive.