http://wiki.nginx.org/HttpGzipStaticModule
Greetings!
According to this page, it would seem that you can not define which mime types should be checked specifically for static gzip separately from the mime types which are compressed on-the-fly.
For more control, there should be something like gzip_static_types.
CSS/JS files are for the most part static, but HTML and RSS files are updated regularly in many situations. It seems like a waste of CPU cycles to check every mime type specified in regular gzip_types
Thanks!