As with so many of Poe's narrators, Montresor isn't entirely reliable. The simple reason for this is that we only have his side of the story, no one else's. Because we only have a partial account of Fortunato's death, we're immediately placed on our guard, asking ourselves if any of this really happened.
Also, we need to ask ourselves why Montresor is choosing to confess his foul, heinous crime now, many years after the event, and to whom. Wouldn't it have been better to have taken such a dark, terrible secret to the grave instead of divulging it? Then again, if we examine the precise details of Montresor's elaborate murder plot, the whole thing's so incredibly gruesome, so totally over the top, that it has the ring of truth about it. Stranger things have happened than the lurid tale that Montresor tells us and his unknown auditor. All the same, and for the reasons already given, it's best to remain wary of placing too much trust in what he says.