Most of the time if your designing with css instead of tables then it is a good idea to go all the way and make your websites standards compliant aswell. In the end you will get:
1. Simplified development and maintenance means quicker production time.
2. Dramatically lower costs of Bandwidth and Hosting.
3. Tremendous search engine optimization and ranking potential. (bots can scan the pages easier)
4. Future-proof.
5. Provides print friendly versions for all pages
6. Improves the User Experience - Pages load quicker and are more responsive, which means that users do not get annoyed waiting for your site to download and go to a competitor's website to find what they are looking for.
7. Increased accessibility for users with disabilities - Content is accessible to a wider range of devices (screen readers, browsers, text based browsers, hand-helds, search robots, printers, fridges etc)
8. The site is not designed for use by web browsers only. It is stable across a wider variety of devices, such as Mobile Phones and PDA's.
9. Any knowledgeable developer can understand and alter code, therefore less dependency on a single developer.
10. The ability to make rapid, site-wide changes for styling and layout.