The place of fate and free will in our lives is an ongoing question in Holes. Sachar has fun playing with the idea of destiny, but he never directly tells the reader what to think. Is the curse real? Does it actually determine what happens to Stanley and his family? Or is it just a funny idea that doesn't have much to do with Stanley's story? Different members of the Yelnats family seem to have different ideas about the curse and about how free they are to direct their own lives. Regardless of what they believe, though, the legacy of the past seems to hold sway over many of the book's characters. Sometimes for the better – but usually for the worse.
The curse is a cool literary device that makes the reader feel like things have worked out neatly at the end of the book. But beyond that, it really doesn't have much to do with the plot; the characters make their own decisions just as they would if there were no curse.
Through the course of the book, Stanley figures out that he isn't just a slave to his fate. By overcoming his sense of being controlled by the curse, he overcomes the curse itself.