Nah, I'm pretty sure the WWE does get guys to memorise scripts. They didn't do it for Austin/Rock, but it's something that's come in with the more modern WWE times.
According to Mick Foley's new book, that came in around about the time he was Commissioner. Also, he says they even have run-through rehearsals for exactly how the promos will go nowadays.
Big names/trusty mic guys probably get a lot of artistic license, though, I expect.