This question comes up from time to time. Personally, I wait until the certificate arrives in the snail-mail.
- The criminal code says you must be able to produce the registration certificate "on demand" by a peace officer. If the police officer believes that's the paper certificate and you don't have it, then you get arrested.
- Since the paper certificate is easily faked, police with access to any form of communication (voice phone, radio, computer) check with the CFRO database because it's not the paper certificate but the database that indicates that you are a 'holder' of a registration certificate. Certainly the opposite is true: if you have the paper certificate in your hand, but the database says that certificate has been revoked, then you are not the 'holder' of a registration certificate according to the law.
- When the LGR ended, the Ontario CFO Wyatt declared that the registration was the certificate number. He then said that make/model/serial/pal/date had to be recorded in his log books for non-restricted firearms sales, but not the registration certificate number which no longer existed for non-restricted. This registration certificate number appears on your transfer paperwork. The minister of public safety told the CFOs they were not to record make/model/serial in the log books, the CFO said his interpretation was the law required it, and the conservatives clarified the law.
- If you phone the CFO's office and they say you can go, then in court you'd have the defence of "Officially Induced Error". But you still get arrested, spend $ in court, and it still could go either way.
The CFO's office routinely send ATTs via email.
Maybe they can send registration certificates by email, although I've not heard of anyone getting one.