How long do you think is best to force a Static HTML page from your webserver to be cached by ISPs proxy server out there? If you do not set anything in your apache, your page will be cache longer maybe 1 months as ISP willing . If a customer upload HTML page to your machine but got stuck to his ISP proxy cache then he can't see the new page and IE5.x is buggy that It did not pass proxy , your customers will be yelling at you , the capaility comparison between you and other hosts occur ,you have to answer more question by phone and email what happen to their sites. But if you force no HTML cache at all, you'll lost more bandwidth and if you use mod_gzip , you will lost more machine power.
