It's easier to true, and certainly to learn how to true, when it's not very far off. Everyone has a different tolerance for trueness. A millimeter or 2 is minor, a centimeter is way too much. Read all you can about truing and wheel structure, because you will best be able to true (or do anything mechanical) if you understand the why, not just the how. Sheldonbrown.com is a great place to start, parktool.com/blog is also very useful. Once you have perused them go to Google to learn more. You will need to learn about hub adjustment and tire install/removal along with truing, as parts on a bike interact with and depend on each other.