Thanks for the insights. I'm still not sure what's the impact specifically for *geany*. I don't know what badness happens if you run geany without creating the icon cache (I haven't noticed any issues with my local) testing. We don't install lots of images at all (4 per size), maybe not worth?
Is "using the cache" something that's handled transparently by GTK or should there be some specific code inside Geany to support that cache file? I don't know about such code but maybe I just didn't came across yet.