If there is a file called under_score and you search for under_score, then you'll find it.
But only a webmaster, indeed the owner, is likely to make that search.
If you search for under score - and the page under_score.htm also features under score, then you may get served under_score.htm
If you search for under score - and the page under_score.htm does not feature under score, then you are unlikely to get served under_score.htm
I'm sure there are many, many times when under_score works; but in general, other forms work better.
But never test any search on a webmaster search; always test with the kind of search a customer would make. They are the ones that matter.
[edited by: Quadrille at 12:29 am (utc) on Jan. 11, 2007]