Actually it's both an art AND a science. And it does take some skill. I've had more than a few "Home trued" disasters come into the shop.

Our manager has anyone that will be touching a wheel in the shop read this book first

http://www.amazon.com/Bicycle-Wheel-.../dp/0960723668

It's a very interesting book, and worth the read IMO. Even if you never plan to build your own wheels. Knowledge is always a good thing.