Your body needs certain nutrients, and any balanced diet will have those nutrients in the right proportions. However, within that general framework is a lot of latitude. Take for instance the "maintenance diet" that I was given when I finished my diet:
- Breakfast: 1 serving carb, 1 serving fat, 1 serving protein, 1 serving fruit
- Lunch: 1 serving protein, 1 serving fat, 2 servings vegetables
- Dinner: 1 serving protein, 1 serving carb, 2 servings vegetables
- Snack: 1 serving protein
- One day a week you have a "free day" followed by a no-carb day
- At least 2L of water a day
A Protein is considered: 2 whole eggs, 5oz land meat, 7oz seafood, etc.
A Fat is considered: yogurt, cheese, butter, seeds/nuts
A carb is considered: whole grains, starchy vegetables like potato or corn, etc.
This is just one of many maintenance plans that will work. Essentially you'll be eating plenty of protein, plenty of veggies which are probably two of the most important parts of your diet. You'll also get your carbs and fats. The free day is meant to throw your body's metabolism off so it doesn't get used to your diet and start storing more. The no-carb day is there to correct for the extra calories you took in on the free day.
Contrast that with the StrongLifts Diet, which is designed for weight lifters. A diet for weight lifters is going to have a lot more protein and fat. Also to deal with the increase of protein is a lot more water (minimum 1 US Gallon a day). On that diet you have 10% junk meals (4 junk meals a week). You'll be eating more, but only gaining weight if you eat a lot more.
So as long as you are getting the protein and vitamins & minerals you need, the rest is up to you. While I have no data to back it up, I think it's a reasonable and good idea to allow for fun meals. If you monitor your weight weekly you should be able to get an idea if your weight is going up or down and adjust accordingly. My personal experience says that if I cut back on carbs for a day I can usually correct a 2-3lb increase (hint, it's not all fat if it came after just one free day/meal). I also find that if I slack on my water intake, my body holds on to weight a bit more. If I did gain a couple pounds of fat after a week on a cruise, I can still correct it easily by cutting carbs (so that my pancreas is secreting glucogon to help burn the fat for longer periods of time).