In your SEO settings you have the option to block spiders from non-critical pages. This serves a dual purpose. First, it saves your server from some pounding it would otherwise take on irrelivent pages (like the "report post" page of each and every comment, the "email article" page of every article and the like). Secondly, it helps spiders get deeper into your site since they aren't drawn off into wild goose chases.

"Non-critical" means everything except these pages:

• any content from index.php (front page, category pages, custom templates)
  
• anything on comments.php that doesn't have action= in it (in other words, the main thread view)
  
• anything on link.php (article details)
  
• anything on memberlist.php (the member list and all member profiles)
  

URL rewriting is a non-factor, as the urls have been rewritten to their php forms before the determination is made.