I would just like to add that you should always remember that a first-person narrator only gives his/her viewpoint of the events. It isn't Montresor's insanity that would necessarily make him unreliable, but the fact that we know only what he is thinking that might make him an unreliable narrator.Montresor says Fortunato has offended him, but we don't know if this is true, or if it is just imagined in Montresor's sick mind. If we look at Fortunato's reaction when he realizes Montresor is going to kill him, he doesn't seem to have any idea of why this is happening. I don't think there's any doubt that Montresor killed Fortunato in the manner he described, but we can't be sure why he feels he's been insulted.

I think you can count him as a truthful narrator whether he is insane or not. As a first person speaker, you are inside his head the whole time. It doesn't matter what his frame of mind is, he is credible because you're learning from him.

Montresor doesn't try to mislead you as a reader, although he's obviously trying to mislead Fortunato. You get to read and enjoy every thought he has while exacting his revenge; this, alone. makes him a truthful narrator.