[schnable]

Static content by definition doesn't change so this rule isn't for that case. Caching benefits even dynamic content, and dealing with cache invalidation is a separate problem than what is being discussed here. Your clients may always want the freshest content, but that often isn't scalable. If it is, you can put no expiration in your cache headers and let 'er rip. But you should still respect the idempotency of GET requests.