Firstly, if you're not using Varnish yet, then you might want to wait to introduce it into your application stack until you really need it. It also depends on what you think is simplest to understand and handle for you: Learning how to cache in your app or introducing a new layer in your application stack. More layers equals more trouble, all other things equal.
Secondly, do it in the way that makes it easiest for you to control the cached copy, so that it does not get stale. If in your app you can cache it in such a way that it automatically expires when it needs to, and you do not have that ability with your setup or know how to do that in Varnish, then do the caching in the app.