As a variation on this suggestion, an additional set of filters could be
provided that indicate that if a file is in the cache, not to make a
conditional get request and instead use the cache. It's a waste of time to
even check the server if the user knows that certain files are never updated,
and it spares the server responding with not modified responses.

Another option would be to use any provided Cache-control or Expires headers
to judge whether to make a request to the server.

Also, apologies for reviving this very old thread, but it is related to what I
was searching for.