I updated my install without any issues. If you're worried you may want to wait a few days and see if any bug reports come in. Otherwise, I think you're probably safe to update without breaking the theme. Be sure though that any plugins you have activated also have updates otherwise you may experience problems on that front.
WordPress will always have updates. Generally minor updates will not break a theme. Major updates like going from 3.2 to 3.4 may break a theme depending on what WordPress has updated. I would say check the forum before you update to see if anyone reports a bug. If none are reported then you're most likely safe.
If you're still worried you may also want to setup a test site.