I really wouldn't recommended boiling the stuff, at all. I've never heard of anybody doing that before but I can think of a few reasons why it seems like a bad idea. The main thing is that fresh earthworm castings are alive; there are active microbial cultures in there which are beneficial to the soil and to the plant. To me and many others the primary notion behind brewing organic compost/vermi/humus/guano teas has much to do with these microbes, and our goal is to allow them to proliferate. Many of the microbes we desire survive or thrive only in aerobic conditions: they need oxygen to survive (like we do).
Boiling water is obviously very hot, enough that it is going to kill a great majority (if not all) of our desired micro-organisms. Usually water will at least have some minute amount of dissolved oxygen in it, and aerobic bacteria will tend to use it up quickly. The act of boiling water actually depletes all of the oxygen that was previously dissolved in it. That is bad news for aerobic bacteria and even plants if they are irrigated with water that was boiled, as plant roots also breath oxygen.
I don't know what boiling fertilizer does to it's chemical characteristics, such as nutrient availability.