I don't really think ANYBODY (except maybe webmasters) does searches with an underscore between keywords (or a hyphen for that matter). It is either just the keywords themselves OR as a exact search with quotation marks around them.
I won't restart the debate here either, but I'd just avoid underscores in general.
But, to each his own perhaps... It's great that GT has added an option to use a "-"!
By the way, my #1 rank at Yahoo (+ Alta Vista/AllTheWeb) for "web site designs" (without quotes) is still holding up, despite the result pages total going down from 54 Million to 49 Million. Hey, I must have done SOMETHING right! ;)