The key here is not going to excess.
The problem with cheating using things like wheat and other grains and sugar and crappy oils is that for a lot of us, they are addicting, and excess is hard to avoid. That, plus everyone has a different level of tollerance for these substances (no, I will not call them foods), that knowing what your personal level of "excess" is would be tough at best.
I would recommend you not consider it cheating so much as a possible carb "refeed" (if you really think you need one) and look towards things like squash, sweet potatoes, carrots, even the occasional white potato for sources of carbs on the refeed days. However, sometimes we don't have these options, hence the 80/20 rule and if consuming grains is the only option I recommend you not feel so guilty about it and just accept it as a reality of modern life. Nobody is perfect, and I hope you don't expect yourself to be either.