A tiny little point, but worth knowing nevertheless: Google says that it is best for SEO purposes to use hyphens rather than underscores as separators in your URLs. According to this SEO article, a Google representative said: “Basically we use some parts of the URL for understanding what the page is about. And the way it works is that we need to be careful about where we are segmenting because many things on the internet, things that people write about have an underscore in them, so we can’t easily segment at underscore and that’s why we are recommending dashes.”
I did say it was a tiny point, and it absolutely is not worth changing anything that already exists. But if you’re confronted with the “Save As…” dialogue box in the future, and the file you’re saving is heading for the web, then you may as well use hyphens to separate words.