Dan01 - 4:07 am on Nov 24, 2012 (gmt 0) [edited by: Dan01 at 4:23 am (utc) on Nov 24, 2012]
It doesn't cost any extra to have an htm at the end and it was a convention I have used since 2000. I don't think NOT having an htm will affect the rank.
As per underscores and hyphens - I use underscores. I have no evidence, but years ago I looked into it and read that the underscore was seen as a space by the SEs. So I still use underscore, never a space and only a hyphen now and then. I now have some WP sites that use hyphens only.
[edited by: Dan01 at 4:23 am (utc) on Nov 24, 2012]