Importance:
LowIt is often debated in the forums whether Google words in file names should be seperated by hyphen or underscores. GoogleGuy confirms here that
underscores are not treated as word seperators, rather they are treated as part of the word. (Note: this is not something new, but since I have read many experienced webmasters claim that underscores do count as word seperators, I have given this quote a 'high' importance level, to help get the word out) 
Link to quote
GoogleGuy Says:
I don't consider that a bug--I think underscores are often useful for phrase matches, and I used one in a product search the other day. Dang'ed if I can't recall it now. So I would stay away from both %20 (ugly) and underscore (treated as part of the word) if you want the url to have separate words parsed out from it.