I have hosed off old AT power supplies with hot water (and a little liquid soap) before. As long as you let them dry out (few days to a week depending on local humidity) for a while they should be fine (and no soap was left inside, flush it very well). I would recommend you don't soak a supply you just had running, let the circuits bleed out a bit.

Its just easier then stripping it 100% to clean dust bunnies. Our local water is not from a well/spring and does not contain huge calcium deposits so anything I wash doesn't turn out white and chalky.