The traditional rule of thumb is to make sure that students are clearly within the instructor's range of vision. Remember that you may be able to manipulate seating to foster any number of effects from closeness to conflict. You'll want to experiment and solicit suggestions from students. For example, if you want to encourage discussion, place desks or chairs in a circle or horseshoe. If you plan to lecture, arrange the furniture so that all students can easily see you without straining.
Good environments are frequently flexible ones.
Feel free to have students move their chairs several times during a class. For example, you might have them move into a circle for discussion, into a small group for in-depth exploration of a topic, and back to rows for your lecture. Experiment with different room arrangements to find those which work best for you. Taxonomy of Educational Objectives. New Directions for Teaching and Learning, No. 2.