Anything that runs gtk-update-icon-cache will do a full global re-cache, so as long as it is run at least once since you install geany to a live system, it will be fine.
Again, the risk of not running it is that cache lookups think it doesn't exist. So, what constitutes a cache lookup? Well, it is built into gtk, you're not supposed to have to think about it. :) Basically any function Gtk offers for looking up icons for you by name rather than absolute filepath. See e.g. gtk_icon_* or gtk_image_*, glade GtkImage* stuff...