Not to scare anyone, but our first cruise, we didn't make the sign up before the lifeboat drill, and our daughter got separated from us. Since she hadn't signed in, we didn't have a pager, and they didn't "KNOW" who she was. It worked out and they got her back to us, but it was 30 minutes and a lot of fears. 2nd cruise, as soon as the clubs opened for registration, WE DID IT! If she had gotten separated again (Yeah, like I was letting her out of my sight!) they could have paged us to say where she was.
IMO it's worth the 15 minutes of time to register your kids early, just for peace of mind. It's also considerably less crowded early. Most people register before dinner on the 1st night which means that it can take 30-45 minutes then, vs. 10-15 minutes earlier.