Here's the problem I was having while working on a custom theme for a client. Certain pages, posts, tag pages, archive pages, and custom pages either had the sidebar, or did not have the sidebar. The main content div #ContentW had a 72% width when the sidebar was present, otherwise it was 96%. The problem was that I dislike having to do things manually when they can be automated through code. What I was having to do was manually add/remove page-specific classes to the css file to reflect whether the sidebar was present on that page or not.