Personally, if I'm going to eat something not primal then it should be absolutely delicious - loveingly homebaked peanut cookie for example. And then it's not "cheating" but simply the non-primal portion of my overall "diet" in the sense of way of eating.
Even if I'm in a situation where I'm certain that what is being served is non primal (restaurant or takeaways for example) I can choose either to fast or to eat it and call it part of my 20%. Either way, it's not "cheating". I don't really think there's any such thing.