For me, the word "cheat" refers to some sort of act of infidelity, or breaking rules for personal gain, and what I eat generally doesn't rise to that level of moral question--with one exception I'll get to in a moment. Our diets and bodies are so fraught with emotional, shame-based language, that I always take a moment to take on that view when this comes up. To me, "cheat" is a few small steps away from shame, which is something people feel when they can't stay on a bad diet that's unsustainable, and they feel shame for their "poor character," or for being too fat, or whatever. An 80/20 diet inherently provides room for 20% variation from some guiding principle, and making those variations strikes me more as treating than cheating.
For me, those treats generally serve to make other foods more delicious, from using a little cheese on something, to soy sauce (gluten free), to eating a muffin made of almond flour. I try to avoid wheat and (unfermented) soy no matter what, so a gluten/soy free cookie might be a treat. I do more of a PHD, so I'm already eating rice occasionally. I might eat corn tortillas in a Mexican food context. I'll eat a product with sugar sometimes, but not corn syrup or agave nectar. If there was a guiding principle to my 20% treat foods, it would be that they still reflect whole foods, minimally processed, that could more or less exist without horrendous industrial processes.
What I do consider "cheating" is the purchase of conventionally (CAFO) produced animal products. I made a commitment to myself, and (goofy as it might sound) to the "universe" that if I was going to eat animals or their products, I was going to choose the most sustainable, least harmful products with the best animal welfare practices. If I couldn't afford those, I would do without until I could. It's an imperfect commitment, as I have somewhat limited access to reliable information. But I do the best I can. And when I do mindfully purchase animal products that do not fit this principle, I'm breaking the "rules" for personal gain (price, social, convenience), and THAT'S my idea of "cheating," because it harms others, be it the animals, the environment, the workers who process the animals, the local pasturing farms struggling to compete with the big industrial livestock business, etc.