So you want your posts to have the actual UTF8 curly or slanted style characters instead of the HTML character entities that are normally replaced for the straight style characters you get when typing content on a keyboard, correct?
Post content is run through
wptexturize() to insert HTML character entities. WP does this by adding it as a callback to 'the_content' filter. You could create your own callback function that in turn replaces the HTML character entities with the actual UTF8 characters in the same manner. Your callback needs to be added with a priority number higher than 10 so that it runs after
Alternately, you could rewrite
wptexturize() so it uses UTF8 characters instead of HTML character entities. Use a different function name. Then remove
wptexturize() from 'the_content' filter and add in your version. This is technically altering core code, but in a manner considered acceptable because it is through a filter and persists through upgrades.