(Popular) rappers spitting true, off-the-top freestyles is a thing of rap's past. Even getting (popular) rappers to spit a pre-written freestyle is a thing of rap's past. When Drake got forced to spit an impromptu freestyle on Hot 97 for Funkmaster Flex, and pulled out that Blackberry, the game changed.
Since then rappers such as Wiz Khalifa, Lil Keke, and Vado have "freestyled" off their phones. The only rule and expectation at this point by fans seems to be "Don't suck." The repercussions of rapping a pre-written or yet-to-be released verse for a freestyle are infinitely less than if you actually try to freestyle and blank or stutter your way through it. Avoid being the next Eli Porter at all costs.