I've seen this question several times before, and each time there are a number of people who hypothesize that having a "stockpile" of CO2 ready and waiting when the lights come on could only help the plants. I don't, however, remember a single person stating that they have clearly seen a difference, much less providing any real evidence that a 30 minute offset changes a thing.
That being said, it's certainly possible that an offset could help. I can't imagine 30 minutes of CO2 without lighting could hurt much (though I suppose it's possible if you regularly push CO2 to the absolute limit). Until I see some evidence, from some legit side-by-side study, I'm gonna have to call this one a myth, or at best an unconfirmed hypothesis.