Sometimes that can bite you in the ass, though. I used to tell people I was diabetic so I could low-carb in peace. Then I became diabetic. But, yes - I'd still totally lie. 'I'm allergic to it', is one of my favorites.
I do have very legitimate reasons for avoiding some of the foods I do, and I will use that when needed. I will lie, too. I will also just say something like 'I don't eat that shit!' if I'm feeling particularly feisty.