The least you should do before a site launch (promoting it with the large public):

• Enable performance enhancing functionalities like CSS optimize etc.
• Check the status report page.
• Where possible activate caching of specific Views and Blocks.
• Test all functionalities available to an anonymous visitors, registered users and the webmaster (your client). Register, create content, signup for the email, test the RSS feed, etc.
• Watch all visitor accessible pages in all common used browsers. Special attention for IE and especially IE6 which is still used more than e.g. Safari (sept. 2010).
• Disable all unnecessary modules like developer modules, views UI, image cache UI, cache disable and other helper modules during development.

To be continued...