It's in the sticky guide above, but there is a lot of information to wade through. This was also answered in a Five on Friday, just to confirm Connor's answer.
From Way of the Samurai: A Guide to Bushido in Ultima Online
# Does Human inherent +20 skill effect my chance to parry with a shield?
No, it has no effect on your inherent chances to parry with a shield, as long as you do not have any real Bushido skill.