> Wouldn't having them already up give a lot less warning in a first strike situation?

Improved first strike capability is worthless íf it isn't crippling, and "devastating enough" first strike capability from orbit is completely unaffordable, and impossible to build up unobserved.

Being in orbit is a hindrance more than anything, really, because maintenance becomes ruinously expensive, everything is trivially observable for all your adversaries and you have to align the orbit with your target beforehand, too (which, again, everyone can observe).

> It doesn't even have to be rational though, combinations of graft and brinkmanship would be enough.

Enough for what? Threatening to nuke some satellites? Because anything else you can do easier, cheaper and on a larger scale from the ground. Why would you bother with nuclear warheads in space when you can just build/maintain like 10 ICBM silos for the same cost?