Yes, actually! I mean, some teachers wing it a lot, but I prepare for every minute of every class, at least until I'm comfortable with the material and the students. Once you're in the situation you can be flexible and respond to different things, but it feels better if you go into it as the expert, like when you're showing people around a place you know really well. See, you're sometimes a teacher already! (Not to mention the gajillions of ways you're a teacher to your children.)