This is good practice. Extra points if you insert them regularly so you can tell when you were owned.

Great idea! Decades ago, we would do this when selling lists of (voluntary) subscribers. We would seed the lists with our own secret monitoring names and addresses in order to track the use and misuse of the lists. It's a shame that these simple things need to get reinvented constantly. Sometimes, people see differences when they should see similarities.

We used those as well.