If you want to maintain the rule you should better invest sufficient continuous energy to keep the rule adherence at a level where it is useful instead of letting it slip and then invest significant energy into getting it back.
I agree with this.
I also agree with you about the importance of making children feel significant. Hand in hand with children expending more brain power on their parents then the parents spend on them, I have a much larger world of which they are just a part, whereas until they are older, I am a much bigger part of their reality. I try to keep this asymmetry in mind and try to be more present for them. (This is one of my biggest challenges as a parent.)
Again you infer too much from my isolated remark that it be risky.
Actually, what I was responded to was more central in your post:
If the children notice this they may assume that you either condone, accept, bear or ignore it. None of these has positive effects.
My point was that I think it has mostly positively effects, because it teaches them the truth. If the parent didn't interfere with the undisciplined behavior (for example, if they are too tired or preoccupied or conflicted) then it is probably the case the parents condones/accepts/bears that behavior at that moment.
A long time ago I agreed that we establish the same rules and back each other on decisions made by the other. But often enough I find myself in a situation where I have to back her - but don't know to what end.
We struggle with this also. For example, with keeping a schedule and with routines. It completely doesn't work unless both parents maintain it, but in our case it is more important to one parent than the other.
In our case it is also more important to one parent than the other. Sadly.
Followup to: Strategic ignorance and plausible deniability
My in-law always says: "For children it is easier be forgiven then to get permission."
EDIT: This post is superseeded by my Book Review: Kazdin's The Everyday Parenting Toolkit I recommend reading only that. The remaining insight of this post is: Children expend more brain power on their parents than the parents on them.
I can say from experience: That is risky.
Children (esp. small ones) expend significantly more brain power on their parents than the parents on their children (your mileage may vary). I can assure you that they will notice these cases - at least some - and take that into account one way or the other.
If the children notice this they may assume that you either condone, accept, bear or ignore it. None of these has positive effects.
Possible alternative strategies:
I am influenced by The Adlerian School. Of relevance here is Striving for significance.
The testing of limits and the resulting interaction with the parent give the child a feeling of significance if the parent acknoledges the act of the child even if he doesn't agree with it. On the other hand ignoring the act of the child is negative feedback about significance.
EDIT: The asymmetry between parents and children with respect to the effectiveness of deniability can be generalized to any situation where one actor has significantly less overall information about the situation than another actor and thus might not be able to reliably estimate whether deniability is possible.
ADDED: tadamsmar pointed out that ignoring is scientifically known to be effective and the advice or rather personal expierence I have related in this post may be contraproductive (at least if applied in isolation).