In "The Cask of Amontillado," are there ways that the narrator might be manipulating the truth?

Absolutely!  The narrator, Montresor's, very first line indicates the heightened emotion he feels surrounding the events of this story and, therefore, establishes a motive for exaggeration or manipulation.  He says, "The thousand injuriesof Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge" (my emphasis).  This kind of exaggeration of the truth is called overstatement, or hyperbole, and it shows us how very wounded Montresor felt.  Overstatement...

Absolutely!  The narrator, Montresor's, very first line indicates the heightened emotion he feels surrounding the events of this story and, therefore, establishes a motive for exaggeration or manipulation.  He says, "The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge" (my emphasis).  This kind of exaggeration of the truth is called overstatement, or hyperbole, and it shows us how very wounded Montresor felt.  Overstatement is used to emphasize the truth, and so we can understand that Montresor felt he'd been injured a thousand times by Fortunato, even though it was likely not that many.  Here, he begins to attempt to justify his murder of the man.  

It also seems as though Montresor has a guilty conscience, as, he says in the last paragraph, that he has kept this secret for "half a century."  To keep a secret like this so long would surely weigh on a person, and the fact that he feels the need to tell it now indicates that he wants to clear his conscience.  Montresor seems to be on his deathbed, confessing his sins to a priest because only a member of the clergy would be likely to know, as he puts it in the first paragraph, "the nature of [his] soul."  His guilty conscience might make him even more likely to paint as terrible a picture of his victim as possible so that he can further justify the murder.

For that reason, perhaps, it is very evident that he dislikes Fortunato, even though he admits that, aside from his one weak point, "he was a man to be respected and even feared."  Thus, if Fortunato was a man who everyone -- including the person who murdered him -- respects and fears, he is likely not as awful as Montresor makes him out to be.  Aside from his pride, he sounds like a decent person.

In the end, Montresor's wounded pride and guilty conscience are both very plausible reasons that he would manipulate the facts of his story.  He wants to make his acts seem as justifiable as possible because he is anxious to relieve his guilty conscience before he dies.