
As a web developer I started to use underscores to separate words from the beginning because of the language: I am generating the URL from the title or the article (whatever the content – no matter if it's a blog, a news portal or an online store) and there are a lot of words that uses hyphens as the grammar requires in the Romanian language (I could give you some examples, but it would be meaningless for the most of you) and even some names contains hyphens.
As I told you in the earlier articles, the best URL is the one that's readable by both the humans and the search engines and it's the clearest and easiest to understand. From my point of view, these conditions are accomplished using underscores as word separators and the hyphens only where they are needed.
Wordpress, for example, started from the beginning with the hyphens as word deliminator and they are stick with it. That's a good thing, even sometimes I am not sure if the names in the titles are of two individuals or one name created from two other names put together.