I'm not sure Nick's storytelling perspective changes the readers' attitudes much at all. What probably does change is our level of awareness regarding the people and events of the story.
Let's face it, almost every story is better told after the fact. From that vantage point, a narrator can tell us not only what happens but has had time and distance enough to also tell us what what it means. Nick acts as both a storyteller and an interpreter of the events which happened that summer.
How does that affect us? We probably understand the context and consequences of those events differently. We probably also see Gatsby as a more sympathetic character. Nick has moments in his storytelling where he doesn't like Gatsby very much; however, all that is tempered by Nick's knowledge that Gatsby dies, unfairly, for having loved an illusion.
It's interesting to ponder how the storytelling (not necessarily the story) might have changed if Nick told it in present tense. Thanks for making me think!