In the opening to the novelization of TMP, Roddenberry (writing as Kirk in the first person) calls TOS an exaggerated dramatization of the five-year mission.
There's also a quote from Paula Block in Voyages of Imagination
(also here: http://www.canonwars.com/STCanon.html):
"Another thing that makes canon a little confusing. Gene R. himself had a habit of decanonizing things. He didn't like the way the animated series turned out, so he proclaimed that it was NOT CANON. He also didn't like a lot of the movies. So he didn't much consider them canon either. And-- okay, I'm really going to scare you with this one--after he got TNG going, he...well...he sort of decided that some of the Original Series wasn't canon either. I had a discussion with him once, where I cited a couple things that were very clearly canon in the Original Series, and he told me he didn't think that way anymore, and that he now thought of TNG as canon wherever there was conflict between the two. He admitted it was revisionist thinking, but so be it.