My own understanding is...
Per capacitor failure, unused equipment is generally good. By being cool, the caps are more likely to be fit and well. Capacitors have other failure modes too, as do other components but I believe high ambient temps, ie appuratus that is on 24/7, is probably the biggest worry.
Per whisker growth, it gets more interesting. They grow slowly, the material gradually restructuring itself over time, more or less, in a simplified view, atom by atom. Not sure if I could find the sources again, but I seem to remember reading that if equipment is ‘powered on’ at the instant a whisker causes a short, the whisker will be just a few atoms wide, and even the tiniest current flow will be enough to vapourise the whisker with no collateral damage. If ‘off’ the whisker gets more of a foothold, with worse repercussions for other compoments once current does start to flow.
My only real experience of tin whiskers is in relation to another recently ressurected interest of mine, restoring old radios. Certain early germanium transistors, specifically those of AF11x type, had fairly pure tin coatings on their cans and are so susceptible to whisker growth to extent that, by now, it is a fair assumption any single AF11x specimen will probably have shorted. These things are now circa 50 years old, although I think they were already failing, less commonly, by mid 1970s. Will modern electronics be as badly affected? Will it take that long? Who knows.
I emphasise, although I find it interesting, I am no expert in tin whiskers. Not sure anybody is, as it seems to be (surprisingly) a poorly understood phenomenon. But I do understand that NASA, for example, are worried about whisker growth e.g. per satellite and spacecraft longevity, in relation to the EU directives of noughties, mandating lead free solder.