It's a matter of opinion, and a "your mileage may vary" type of thing. I have "cheated" or gone off plan for a day or two here and there and i didnt not regain anything except a temporary pound or two that fell off as soon as i got back on track.

I think it's important to consider whether or not you can stop once you start. For me, I had a brief cheat phase of a day or two, and found myself disinterested in carbs much faster than I would have been in the past. I was ready to go back to our WOE without regret.