Well, almost never… I talk about the importance of page names on my SEO page. The trick is to design your site so that your page names and content are tuned for the top phrases in your industry. But what happens if you inherit a poorly designed site, the content becomes obsolete, or a generic term like “MP3 player” is suddenly secondary to a more popular phrase like “iPod”?
In those cases, you create new content and use a 301 redirect (moved) to ensure any traffic to the old page is redirected to the new page. This is critical. Unless the page has no traffic and no incoming links, you should never just delete (or rename) a page and move on.
Here’s a good article on 301 redirects and how to implement them.