Most SEOs know that if you start a new site, the URL structure should use hyphens to separate the keywords and not underscores. So for this URL for this blog post go with google-doesnt-care-about-underscores-vs-dashes-in-urls and not google_doesnt_care_about_underscores_vs_dashes_in_urls. Got it?
Most SEOs also know that changing URLs for new clients from the underscore to the hyphen is probably not something they should do. In fact, all my legacy stories have /archives/[article-ID].html format and I kept them, I left them as is, for example /archives/014445.html. All new stories on this site use hyphens, which I can control.
John Mueller of Google said in a Google Hangout on Google+ at the 21:41 mark "we [Google] don't really care about underscore versus dashes."
He added:
So that's something where I wouldn't bother a setting up redirect for something like that. If the site is set up on one way and using the other way would be nicer, obviously you could do that. But I don't think you'd see a significant kind of positive effect in search from that alone.
Now, back in 2007 Matt Cutts said for new sites, go with hyphens but don't change your old URLs. Again, that advice has not changed.
Here is the video embed:
Forum discussion at Google+.