Is it recommended to call Disposable.dispose() as soon as a subscription completes work?

No, you shouldn't.When an Observable is completed, Observable is disposed of by itself.
This is part of the Observable contract:

When an Observable issues an OnError or OnComplete notification to its observers, this ends the subscription. Observers do not need to issue an Unsubscribe notification to end the subscriptions that are ended by the Observable in this way.