Default is No. I think most people leave it at no because it can cause load problems if turned on.
Quote:
If you enable this option, no-cache HTTP headers will be added to each page. These cause full page data to be re-requested from the server every time the user gets a page, which will increase bandwidth; it may additionally cause an increase in server load as it will cause more pages to be served.
Well I have it turned on by default. I was worried that by turnnig it off people wouldn't see new posts when clicking into a thread. Can I just switch this off?