One cookie isn't cheating. A dozen cookies is cheating.

Seriously, I've always been taught to think of good nutrition as what you do 85% of the time and you're suppose to enjoy your food the other 15% of the time. For my eating, I allow one "cheat" item daily as long as it is just one portion and fits into the rest of my eating to make up the right amount of calories, carbs, protein, and fat. Tonight it was ice cream. I had a half a cup. I am still in my zone for calories and at only 18% fat for the day. I'm allowed between 15 and 20% fat for a day, so I could technically have another 1/2 cup of ice cream and still be in the zone. I'm sure if you count your calories, carbs, protein, fat, etc. that one cookie didn't throw you over either.