I've posted this before, so ignore if you've already read it.
If your children are young, it pays to get them registered early, because then you get your pager. Our first cruise, we didn't get to the lab to register our kids early (DD had just turned 9). DH took them to the pool while I unpacked. Just before the lifeboat drill, DD got separated from DH. While the drill is going on, all services are closed. No one could help us find DD. She was separated from us for probably 45 minutes and we had no clue where she was. They had taken her to the kid's counselors, and once the drill was over, they looked us up in the computer and brought her to our room. If she had been registered, they could have paged us with her location, and even if we couldn't have gotten her, we would have known she was safe.
Especially now that you can register in the terminal, I'd strongly suggest doing it just for the peace of mind.