You may be trying to decide if you should use dashes or underscores for your redirects in mod_rewrite? Apparently MovableType uses _underscores_ and WordPress uses -dashes- to separate URI title words. Actually, you can use either one for either of the weblog systems but those are the defaults. So which is better where Google is concerned?
John Gergye at Li’l Engine performs an experiment using search-words-with-dashes and search_words_with_underscores. His conclusion, which I believe is conclusive, proves that -dashes- are ranked as if you used no character between the words. The reason is Google treats a dash as a delimiter or a space, but Google does not treat underscores as delimiters, just another character. This could hurt your ranking unless someone is using an underscore in their search term.
When creating your rules using mod_rewrite be sure to use -dashes- as separators and NOT _underscores_.