Swanny007 - 5:05 am on Jan 7, 2013 (gmt 0)

One of my top pages uses an underscore, but it's been around for about 8 or 9 years.

I don't use underscores anymore though, I use hyphens instead. If you're moving forward, use hyphens. If it's an existing page, don't worry about it. Or change it and 301 to the new URL, no big deal.