Google's PageSpeed plugin suggests that a website should include Last-Modified and ETag headers:

Specify a cache validator "Resources that do not specify a cache validator cannot be refreshed efficiently. Specify a Last-Modified or ETag header to enable cache validation"

However, Apache suggests that by not including them at all, we speed up websites by eliminating If-Modified-Since and If-None-Match requests:

http://www.askapache.com/htaccess/apache-speed-last-modified.html

these are in direct opposition - which should be implemented? I'm leaning towards Apache's suggestion, as when I want a file cached, I don't want it refreshed.