Part of the original intention of the post was this. I know a couple of grad students who are entering the dissertation phase swing by periodically. Generally grad students view the defense date with this great sense of dread. They see it as a day where their fate hangs in the balance, the day that decides whether they become a Ph.D. scientist or not. However, I think, under most circumstances in science grad programs, the truth is your fate has already been decided before you set foot in that room. At least, this has been my experience and the experience of basically every Ph.D. I know. You may have a committee member that gives you a hard time, maybe more than one, but they are usually the ones that have done so in every committee meeting you've had.
You should absolutely prepare for your defense, polish your presentation, practice. You're talking about something that you've invested in for years, something that you are the world's expert on. You should take pride in your work, and part of that is presenting it to your colleagues in a polished, professional manner. You've been working toward this for a long time. Don't forget to enjoy the achievement.