Speaking to The Huffington Post, the Twilight star pointed out that the moments make sense in the context of the movie.
"Actresses love to stand up and say, after they've shown their tits in a movie, that it was done tastefully and that it was, you know, far from gratuitous," she said. "I mean, projects that really require it are really few and far between. And I think that in this case, it needed to be."
She continued: "This book celebrates being alive and it celebrates being human, and if you want to cover up and deny any aspect of that, you are denying the spirit of the book. I think that it would have been so wrong to shy away from anything in this movie. I think that I would have gotten flak for that. I think that it would have been that I was scared to disappoint my Twilight fans or something."
Stewart added that she doesn't think it was particularly courageous for her to go nude in the movie, saying: "I do hate also when people go, 'Oh, wow, great performance. So brave'. Oh, because I'm naked? That's very annoying."
She explained that it's "really not that hard" for her to decide on which roles to take, saying that she does not approach parts "pragmatically" or by wondering what people will think.
"I understand when people are already successful, you try to control some perception or you try to choose parts based on some expectation of what people are going to think," she said. "You're clearly doing things because you want to be in some position of power and fame, which is not why I do what I do."
Stewart added: "You read material and it provokes you on some level, and the reason you make the movie is to find out why it made you feel all those things. Those things are so rare to find that if you start also considering what people are going to think, you'll never make a movie."
Stewart previously revealed that she feels "really proud" of her nude scenes in On the Road, which opens in the US on December 21.
Photo gallery - On the Road stills: