I allow myself off-plan days. If this is a lifetime commitment, I have to be able to do this for the long run. I can't completely swear off some goodies now and then. If I keep my on-plan days just below my calorie target, my off-plan days don't mess up my whole week. It all averages out.
Some people are more strict --- because they know that one off-plan day will turn into two days, then a week, then a month. As long as you know yourself and know you can get right back on the horse, I say, let us eat cake! In moderation.