Nick is not mature enough at the beginning of the novel to present an accurate picture of himself. His boasts of being the most "honest person he knows" and having come from a background where he learned not to judge others prove to be ironic (if not hypocritical) before the novel's end.
Though Nick initiates his narrative by proposing to abstain from judgement, it is his judgements that characterize the tenor of the story. Nick's views on Daisy and Tom, from the novel's outset, create a moral atmosphere wherein judgement is rather constant.
This fact makes Nick's other statement about himself suspect. He cannot be truly honest if he so quickly abandon's the background he has associated himself with. In this way, Nick a somewhat like Gatsby, willing to idealize his past to present himself more positively in the present.
However, Nick is honest enough as a narrator to change his views. He is consistent in the amount and type of information he shares about everyone one in the story, from Jordan Baker to Jay Gatsby.
This makes Nick reliable, despite the fact that he is not completely honest. We can trust that what he says about others, in judgement, is accurate in his view. He does not tend to exaggerate or to lie about his perceptions.