It would not hurt to email your professors to let them know you will not be in class. Doing so will show them respect for the class and for their preparation. My professors all requested that the students email whenever they would miss class or would be unprepared. They kept lists of who they had called on, and generally chose ahead of time who they would call on that day. If they call on you when you're not there, and you didn't tell them first, it's confusing and wastes class time, and frustrates the professors -- especially the first week of class, when they generally don't expect anyone to be absent.
And, if you email your professors first, even before classes start, you're making a good first impression -- that you're considerate and professional. As a previous poster said, most professors will understand. Definitely let them know.