A new Google Webmaster video highlights how Google currently handles dashes and underscores, and also that how Google and Bing differ on this bit of SEO minutiae.
The video explains, it's still not a done deal … and may never be. Google's Matt Cutts says there's still a difference between how Google treats underscores in URLs versus how it treats dashes.
In the video, Cutts explains that, when Google sees an underscore in a URL, it joins what's before and after into one term. "We still join on the underscore and separate on the dash," he says.
Cutts cautions webmasters against rewriting all of their existing URLs just to have dashes rather than underscores because the actual ranking impact is minimal:
It doesn't make that much difference. It's what we call a second-order effect. It's not a primary thing that makes a huge difference.
In the video, Cutts says Google doesn't have any employees working on changing how it handles underscores and dashes in URLs.
What About Bing & Underscores/Dashes?
For bing it makes no difference to Bing whether you use underscores or dashes in your URLs.
A Bing spokesperson said in an email:
We do not differentiate between dash and underscore in our URL ranking features.