The Green family lives in Denver, like several generations before them, and it's the only home Toswiah/Evie has ever known. It makes sense, then, that the Rocky Mountains would be a major visual symbol of home for her. They even play into the design of the book: Breaks within chapters are indicated by a little graphic of mountains.
In happier times, Mama would sing a song that goes, "We come from the mountains, we come from the mountains. Let's go back to the mountains and turn the world around" (1.29), indicating that the mountains are not only important to Toswiah/Evie but also to her whole family. They are where they're rooted, where they "come from."
In the safe house, Toswiah/Evie says, "I knew we were still in Colorado, because I could see the mountains" (9.1). The mountains provide a way for her to get her bearings. They serve as a reference point, and in the safe house, the fact that they are still visible indicates that Toswiah/Evie has not yet moved on to her new life. As she tries to adjust to her new life, Toswiah/Evie thinks about home: "Denver," I said to the this-place sky. "It's pretty there. We have the Rocky Mountains" (12. 33). She still clings to the life which she's always known.
As Toswiah/Evie uses running to cope after Daddy's suicide attempt, she imagines she is running toward Colorado:
In the distance, I can see the Rocky Mountains, Lulu's smiling face, my grandmother holding Matt Cat in her arms. (25.14)
She runs past all this, though, toward Daddy in his hospital gown. And when he does, it indicates that she is realizing what really matters: Daddy's life and her family. The Rocky Mountains may represent home for much of the book, but in the end, Toswiah/Evie comes to understand that her true home is her family.