The narrator keeps with the attitude of detached disbelief when observing the house and occupants and also when listening to Roderick's story. With any good story, the narrator is not always the most reliable (take Poe's "The Tell Tale Heart" for instance). In this case, the narrator is an adult, aware of his surroundings, seeminly in control of his faculties, and not influenced by any substances which would blur his vision or understanding. In many gothic stories, the need to make the story seem unbelievable and archaic (it happened long ago, perhaps I don't remember the facts exactly) giving a blurred and vague representation of the situation is just enough doubt for the reader/audience that it MIGHT just have occurred. Interesting...
This is a good question for the discussion board since you will get different answers from different people here.
Personally, the narrator's objectivity was never in question for me. The narrator seems to be completely sane and of sound mind and someone who was able to distance himself enough from Roderick to be able to fairly assess the situation. It is clear he cares about Roderick's state of mind, but the narrator does not get emotionally involved and let it cloud his judgment, in my opinion. He presents the story in a matter-of-fact way and is clear in his details and descriptions.