Valleys of rationality are never a reason to call off your own experiment.

If you have some reason to believe that you're in a valley (because they seem common or because someone trusted told you, perhaps), then simply act as if you hadn't entered the valley. Once you're convinced that you found your way out, you can use your new found knowledge.

If you don't think you're in a valley, then you'd think it's stupid to use your old decision making algorithms.

If you're talking about small influences on someone else considering progressing, then you just have to decide how much of your effort to use on warning of upcoming valleys. I suppose its possible that the person/organization in question can be persuaded to try/not try to become more rational, but not be persuaded about the reality/severity of valleys, but I don't think that's a common case.