I used this excellent post Website image caching with Apache to define my header expires. And I have looked at numerous other examples, and they all pretty much outline the same; enable for images and css, disable for php, python etc.
But I am a bit ambiguous about JavaScripts and HTML. Can I add header expire for those as well? Without compromising the live content of the site? I am running Simple Machines forum.
By top eight mimetypes by use look like this, so there is something to gain:
php 46.5 %
png 22.9 %
js 6.5 %
html 5.9 %
gif 5.8 %
css 4.8 %
jpg 4.7 %
jpeg 2.6 %