I'd say that even if the content changes every five minutes, it's in most cases worthwhile to generate a static HTML page every five minutes. About the only time where that's a bad idea is if all of the following are true:

• Generating the HTML page consumes a lot of resources.
• You get less than one hit on the index page every five minutes.
• The server is busy doing things other than serving the index page.

Abigail