I think a lot depends on why exactly you're doing a 'motherboard wash' in the first place. A lot here seem to favour the distilled water route followed by a dry with a hair dryer - personally I don't really see what that achieves apart from giving you a nice sparkling clean motherboard - perfect if you're mostly concerned about presentation.

I prefer the Isopropanol Alchohol (purest is best 99%+) route principally because I want the electrical contacts to be clean - I'm not that concerned about the overall appearance of the board and Iso is an excellent electrical contacts cleaner.

That's my view for what its worth - I'm sure others will have their own views and reasons for whatever route they favour.