I also use to fret, just a little, that leaving the capo on might - well - fret the frets just a little, or something like that....
But I'd forget and leave it on anyway - sometimes for days at a time - and have never noticed a problem. I imagine that leaving the strings fractionally more stretched than usual might have some tiny effect, but then so does stretching and rubbing the strings while I'm playing, or putting the capo on for an hour or two when I'm using it. So might leaving it in an alternative tuning, etc. Most things you do in life have some small effect, but it's usually not worth worrying about. Just walking to the fridge to get a beer probably fractionally wears my knee joints, but I don't lose any sleep over it.
I have several guitars, including a couple with nylon strings which don't get played much. One sits on a stand and the other on a hanger. And just occasionally, one or the other will quietly break a string, without having been touched at all! Usually the D string. No matter what you do, or don't do, strings won't last forever anyway.
So I don't worry to much. I live with whatever happens, and whatever's convenient, and just change the strings regularly every couple of years -whether they need it or not...