I wouldn't put the word 'Reward' on it. No way! That's just asking for the dog to be stolen.
People do it all the time, steal dogs...wait for the owners to offer a reward and then "Hey, guess what? I found your dog."

Sad but true. By putting the word 'Reward' on his tag, you're telling everyone that yes...if you steal my dog I'll definately give you money to have him back.
Cass.