When to use .dispose()

Question

I have a blocking context that subscribes to Observable.(I know it is a bad practice, but that is how it is)

 publis String someMethod(List<Entity> data) {
   save(data);
   return "success";
 }
 public void save(List<Entity> entities) {

    Observable.fromIterable(entities)
            .forEach(entity -> repository.save(entity));//as I understood forEach will automatically subscribe
}

I have heard that you can do .dispose() to release threads, but not sure if I must do it here and if I do, shall I just dispose or do some checks before? Or maybe I should just use .block instead?