I am also paranoid about ‘trusting’ google by putting all my sites into one account. They are in it for profit, not goodwill! The advantage of cross linking from different websites and hosts seems to me to be negated if you put them altogether in one webmaster account. They’ll probably figure it out anyway eventually, so why make it easy for them.
I view sitemaps as another possible page link in the google system. If they view it important enough to list them, then they will. I’ve seen a well ranked site where google even included nonsensical (to me anyway) chat conversations.