Whatever eating plan you are on, there are probably foods that you aren't supposed to eat, or that you aren't supposed to eat regularly. I don't mean foods that are difficult to prepare or too expensive to eat regularly, I mean foods that aren't exactly healthy for you to eat on a regular basis for one reason or another. We often think of these foods as cheat foods.
Various diet gurus recommend indulging in cheat foods or cheat meals or cheat days in various ways. There are basically two reasons to cheat on your diet.
The first is psychological. Many people feel that it is too hard to accept an eating plan which forbids some particular food (either type or quantity) forever. If I tell you that you can never again eat bread in any quantity, it might be harder for you to accept that than if I said you could have bread once a month but no more often. Some people feel that having one meal or day every week or month or whatever where you can indulge in foods that aren't good for you acts as a sort of release. Plus, some people might use the thought of a promised cheat meal as a way to help discipline - "I'll skip this donut today, but on Sunday I'll eat a bowl of ice cream." Another reason to cheat is social - you're out with friends, and wind up at a restaurant that doesn't offer good choices of food.
The second reason to cheat is physiological. If you stick to a strict diet, especially one where calories are restricted, fat loss will stop after a while, and you might experience a host of other symptoms - reduced metabolic rate, some hormonal suppression, etc. Many argue that an occasional day or two or more of higher caloric intake will re-set the body's systems and actually make long term fat loss easier. I have my suspicions about this - I'm not sure a short term binge actually contributes to fat loss, as the effects may have more to do with water retention than actual changes in dry fat mass, but I can't be sure.
I have doubts about both reasons for cheating, to be honest. I often find that "cheating" doesn't alleviate the pressure of dieting like letting steam out of a pot, but rather leads me on a downward spiral of binging that is harder to get out of. This is very much a personal observation.
If you are going to cheat, for either reason, there are a few things to consider. Determine for yourself if you have trigger foods that lead to more binge eating. For example, if I eat a bunch of dark chocolate, I can stop there. If I start eating foods made from wheat, like bagels or pizza, I end up on a three day bender. Whether this is for chemical or psychological reasons I couldn't say for sure, but it's a pattern that's been repeated often enough for me to recognize it. What that means is that if I'm going to "cheat" I'll overeat on foods like chocolate, chips made from root vegetables, and even sugar, but not pizza or bagels. That way I can usually control the cheat meal and contain the "damage." You need to determine if you have any danger foods - foods that lead you to binge further - and avoid them, but cheat on other foods if you feel the need.
If you decide to cheat, minimizing the damage can be tricky. Overeating on clean foods is probably the safest way to go - adding 500-1000 calories of good, paleo food might actually do your body some good. If you can't manage that, there are better and worse choices.
The game of "which is worse for me?" is pretty complex and all too often we have to admit to not having any answers. Stay away from trans fats in any quantities. Small amounts of sugar (say, less than 20 grams or so) are probably okay, when eaten very infrequently, but the jury's out on larger amounts. Artificial sweeteners are all over the place - if you need to drink soda, are you better off with Diet Coke or regular Coke with high fructose corn syrup? You'll find people to argue either way, I'm sure, and I can't find a convincing answer. Neither should be part of your everyday diet, and neither will kill you instantly if you drink them on Christmas.
You can make some comparisons. As far as grains go, wheat is worse than beans which are worse than white rice or corn, as far as I can tell. So if you're going to binge on tortillas see if you can get ones made out of corn flour instead of wheat. If you're going to eat dairy, cream and ghee are better than butter which is better than raw milk which is better than pasteurized milk. Dark chocolate is better than milk, the darker the better - 85% dark chocolate has so little sugar I'm willing to have a little bit of it on a daily basis and don't consider it cheating (when I cheat I'll eat 70% dark). Fruit is better than unsweetened dried fruit which is better than sugar-added dried fruit. Erythritol is better than sucralose which is better than nutrasweet. Sugar is better than high fructose corn syrup. Agave is probably equivalent to high fructose corn syrup - the industry trying to tell you that agave syrup is good for you is the biggest scam since soy. Pigging out on grain fed beef is still better than chowing down on the grains directly. Tequila is better than beer (beer often contains gluten).
The question with a lot of my favorite cheats depends on the relative danger of artificial sweeteners - I just don't know how damaging erythritol is, for example, so some treats seem okay but may not be. Take this recipe for example: take a can of coconut milk (not light, regular coconut milk) and blend in 1/3 cup cocoa 1/2 tsp vanilla and 2/3 cup erythritol and 1/3 tsp xanthan gum. Refrigerate for several hours. Eat with spoon. It kind of tastes like something between chocolate pudding and mousse and it's actually delicious. You could sprinkle on cocoa nibs or something if you wanted. Is this safe for everyday consumption? I'm not sure. Is it better than regular ice cream? Yes, unequivocally. Another good ice cream substitute is coconut milk ice cream, but most of the commercial brands I've found are sweetened with agave, which is like hepatic insulin resistance in a bottle.
The final words on cheating: Cycling calories (eating more some days than others) is probably good for you and, if planned, isn't cheating. Eating unhealthy foods isn't good, but you have to decide for yourself if you "have" to do it. If trying to "never cheat" drives you to giving up your healthy diet completely and going on a McDonald's bender, then planned cheats may be good for you. If planning a cheat meal or day every so often makes it easier to stick to your eating plan the rest of the time, then good for you. Figure out how your mind works and go with it. If you must cheat, minimize the damage - try overeating clean foods, and if that doesn't cut the mustard, go for dried fruit and dark chocolate instead of pizza and beer.