One of the first things I do when creating a new site is to set the Administrative Theme (admin/settings/admin) to Garland. Two of my main reasons for doing this are:
- Customer-facing themes are often "heavier" (more images, more CSS, more complex layouts) than core Drupal themes.
- Garland is a fluid-width theme. Having this flexibility is key in the admin area especially when the main theme is fixed-width.