Many website owners might not realize this but utilizing underscores or dashes in your URL structures can make a significant difference when it comes to how search engines read your and display your site. Now with that said don’t panic and realize that you have to go re-write all your URLs because you could do more damage than good.
If you were ever wondering which is better to use your prayers have been answered by Google. In this Matt Cutts YouTube Video he discusses when you should use dash marks and when you should use underscores when writing your URLs. Both have different uses but ultimately lead to the same goal if that makes any sense.
Check out the video and you will see.