Well, mermaidgrl, I don't know of any PLAN that allows this, but it's simply a way for some people to rationalize falling off the wagon.
I think it depends on the person. Are you someone who can walk away from cake? Pick the croutons out of the salad or eat them "because they're there?" Spend half the night tossing and turning over the half pint of ice cream calling your name from the freezer?
If you're a girl who "cain't say no" any indulgence can turn into the "unhinge your jaw and tip back the refrigerator" kind of evening, and that is definitely a bad idea. This can take a week or more out of your schedule, trying to get back on track, and that's just one evening. All too often, folks here relate how that kind of thinking has resulted in days, weeks, or months of lost ground and gained pounds.
So ask yourself what you are really after, and make your decisions accordingly.