Most of us use wordpress because we now Google loves it :) If you did not know this, you just learned :P and if you do not trust my words, just install other blog engines or your own design and publish them at same dates. After 15 days check your pages Google positions! Or just look at your visitor count! …
Anyway … there is a very basic Google advice that says use breadcrumbs navigation. When you use breadcrumbs navigation Google easily understands your hierarchy. And gives you more rank… Also your visitor surfs better too.
You can find this information also in Google’s starter guide for seo pdf which you can find at Google webmaster central.
To sum up, I write this article because this navigation is not included in default wordpress. You need to install a plugin for breadcrumbs or edit the “single.php”. On that page just add this code above the post title.
Every plugin is an extra cost on performance issues. So just change your single.php and do not install a plugin!