It's important to let biscotti rest so that you can easily cut it into pieces. If you don't let it rest then it will be very warm dough and you won't be able to cut it cleanly as the dough will just break. So, even if you let it rest for 10 minutes and you cut it and it's not cutting clean, feel free to let it rest for 5 additional minutes. It should cut easily and clean when you slice it up.