Why nag the user any sooner than necessary?
A user will be more comfortable if the request for permission comes when they actually try to do something they know requires access to the event store.
If you request permission too soon, the user is going to wonder why the app is asking when they haven't even done anything with the app yet.
And what if your app needs access to multiple sources such as camera, photo library, contacts, and events? It would be terrible to just nag the user over and over for all of them up front. Only ask when the source is actually needed and the user probably fully understands that the action they just took (like take photo) requires the specific permission.