This presupposes an element of rationality by both, or all, players.
Are you talking about what you would
expect someone to do, or what one
should be doing? In this case, I think one can act rationally no matter what one thinks the opponent would do.
Ballistic missiles are hard-wired in our psychic as deliverers of doom and gloom. If the enemy has the means then you cannot assume a rational "let's sit and wait."
But you can! If it turned out to be a nuke, you act accordingly. If it turned out not to be a nuke, you act accordingly. Nothing you can do at that stage would affect what the missile would do to
your territory. Whatever it does, a single missile would not materially affect your war fighting ability, so no action has to be taken before impact.
Consequently, you can be rational, even if your opponent is not, if the scenario is a single incoming missile (or even a couple missiles). You can go to whatever your national equivalent of Defcon 2 is, you can start generating if you have a bomber force, you can start dispersing whatever should be dispersed, you can do whatever it is you need to do given what particular nation you are running, but you don't need to launch until after impact.
I think it's far too simplistic to assume everyone would automatically go dumb as soon as nukes are involved. Yes, things were scripted, but there were several scripts to choose from, and all had a play button that had to be pushed manually. Nothing was automatic, right?
What one would not do, however, is to launch a single conventional missile against a nuclear armed opponent with an early warning capability, because
then you would have to assume your opponent was doing what you would consider rational. But maybe that was what you meant all along...