I've been working on the Drupal 6.x dev version of the Andreas03 theme (http://drupal.org/project/andreas03) for which there is no left sidebar region. I was kind of stuck on the regions where on a fresh installation, enabling the theme automatically put the navigation/login in the header by default.
This kind of sucked and would be confusing/interesting for those installing the theme for the first time. After doing some reading and searches on the subject, for which there was very little information, I found and answer. The navigation and login blocks, which are enabled by by default, appear in the first region listed in the theme.info file. So rearranging my region declarations fixed this screwy problem. Another "makes sense but not obvious" moment.
I'm assuming then it is correct practice to list the first region as the default, or where the aforementioned blocks should appear.
So, anyhow. This is all fixed up -- committed the 6.x release and branch you can check it out at http://drupal.org/project/andreas03