Lets take a moment to talk about Underscores vs dashes in URLs, what you should you use, and about the differences between the two. There obviously still are a lot of sites that uses underscores, despite Google recently discouraging their use.
Basically its bad to use underscores, because Google doesn't treat them as word separators. Using dashes, something like the below, will be treated as four separate words.
While using underscores like below will be picked up as one word, and as such may get poor rankings and less clicks.
We also took out the "in" word, not that it matters much. But in some cases you might want to take out common words, because they lower the relevancy of the URL. Therefor its best if you have a system that gives you full control over the URL. Don't just generate the URLs from the page title.
Of cause this is mainly for SEO purposes, so you could basically use whatever you want as URL separators.