As I sometimes say here, inaction is also action. One does not need to take some type of action just for the sake of action or to fill in the void. Silence can be as equally powerful as taking some type of action. It is all ebb and flow like the tide in concert with the various moon phases.
Now as for when NOT to validate, here are some examples that come to mind:
-your spouse is being cruel and behaving like a jackass -when you don't necessarily agree or share their view(s) -they are in the entitlement phase in their A's with OM/OW -they are in the full-on spew rage
It all depends on content and context of the situation on the ground. For instance, when your spouse is in the very early stages of his/her affair where their fog is at its thickest and the dopamine levels are at its highest range, it is VERY, VERY difficult to validate effectively because, in their mindset, you re the EVIL one and nothing you say will get through to them. This is when one drinks the STFU juice in copious amounts.
I've noticed, based on my long experience here, is that one can begin to validate when their spouse begins to voice their confusion or frustrations about the M several weeks/months after the initial BD. Their confusion is a good time to start validating because you've been GALing and staying out of their way. This is when they begin to see internally the contrast between you and their affair partner. Mind you, they'll NEVER voice it during their affair phase because it's akin to admitting that they're WRONG. Not happening!
One only needs to look at Train's threads over in the Infidelity forum to see this interplay quite clearly.