In my experience, I think it is key to only be strict in regard to your prohibitions (& must-eats).

For instance, if my prohibition is specifically against fast food restaurants, a bacon cheeseburger at a fast food place is 100% forbidden, but a bacon cheeseburger at a casual dining ("sit down") restaurant is okay.

In this way, while the caloric intake might be the same in either case for that particular meal, over time I cultivated a habit of not eating based on convenience and impulse and I was able to generally better plan my meals and gain dietary discipline.

Anyway, to your point, I agree that some exceptions are generally a good rule for dieting -- my strategy was to make everything an exception, apart from 3 or 4, 100% compliance* rules.

(*Disclosure: I did drink one soda that year. It was a business meeting where my job success [sort of] literally depended on me accepting a rather insistent potential client's beverage offer to keep the meeting headed in a positive direction...so I did. And I still didn't get the deal. Dammit.)