Come on! stop the paranoia about CPU temperatures and enjoy your rMBP, if it get toasted Apple will replace it for you, if you spend a lot of money on a rMBP you'd better focus on doing something productive on it.
I can't be 100% sure, but for some of my long-time simulations I encountered memory-related errors. It may happen when IMC is locally overheated.
Modern CPUs won't fry even without heatsink. It seems that other nearby components with accessible sensors aren't subject to temperatures higher that +30C to ambient. Airflow is decent either. Therefore I'm not concerned with reliability, but rather random glitches that may occur. As a rule of thumb you should never use CPU operating at TJMax temperature, even though it won't damage itself.