For static sites, if the content of your site has changed then the visitor should see the latest content. I THINK their browser will compare the cached doc time stamp vs the time stamp on the file on the server, if they're the same then it loads the cached doc otherwise it grabs the newer version off the server.
For WordPress sites the index.php is being generated dynamically so it will always load from the server...
To reload a page at a set in terval add this to the head
<META HTTP-EQUIV="refresh" CONTENT="XX">
change XX to the number of seconds you want to to wait between refreshes... Don't know how that would be of benifit to most sites (maybe stock tracking)...
Or you could put a link on the page telling them to refresh it at their leisure with something like this
or you could do it with a button....
<INPUT TYPE="button" onClick="history.go(0)" VALUE="Refresh">