I don't know if this is exactly the number one rule, but it sure ranks high up there. In as much way as people are entitled to their own opinions, I came up with this "rule" because I think it makes complete sense, and it often comes up when talking about computers to someone as eager about them. It basically comes down to preference, but anyway, here it is:
Rule #1: A nerd has their computer set up how they want it. Do not try to change their setup, because it belongs to them, therefore they are correct.
Basically, when it comes to computers (and nerds), we all have a different way of setting up our computer, whether it be our operating system, antivirus software or web browser, amongst many other possible preferences. Each person has their particular settings and programs for their computer. You have your own. Don't mess with the other person's computer, because they have it set how they want it, and you have yours set up how you want it.
Somewhat contradictory to that though is when we have to tell our families how to do things right. Ever have that feeling of needing to be a control freak over your computer at home, because else the family will destroy it? Well, I do. They don't know much about computers, so it's up to you to school them. It's simple, but over time, they learn more about computers, and they get used to your setup. You end up training them with your particular setup and preferences. This CAN confuse the more technologically-challenged, however (such as my own family).
So at the end of the day, it all boils down to preference, but I think that in the same way as dismissing or insulting another's opinion, when it comes to a nerd and his/her computer, you shouldn't mess with it. Leave it the way you found it. Unless you want to keylog their computer and steal their internet banking details; then I won't stop you, but don't blame me if you end up in jail.