The callback is only used when the App detects there is no valid session for usage. At that point you initiate the oAuth cycle and the callback is invoked. At that point a session is created in your App and you do not need to invoke the callback cycle any further since you’re checking for a valid session.
So yes, you’re right, it is up to you to determine if a valid session exists, internal to your App but there are no useless calls involved in this pattern.