If it is like Singapore & Malaysia you dont show anything the IO will know if you are registered or not. This should hopefully end up like Singapore and Malaysia where they are automated entries as well now. You got to look at the bigger picture
Lots of people are having difficulty getting back to back visa exempt tourist entry. You will not and can get nearly 9 months in Thailand if you work it out correctly
if its anything like singapore or malaysia which has had same system running for a while. There is always travellers who havent done it and have to do there and then. Also go to back of queue while waiting for the confirmation mail. Which in Singapores case is about 5 minutes