I just started using a P30 back this week. Is using the Expo disc the best way to set your white balance?
It depends on what you're shooting, and why. For portraits, especially of Caucasian women, a warmer-than-accurate white balance is appropriate, and for landscapes, a technically accurate white balance will in many cases ruin the ambience of the image. But for product, catalog, and fashion work, where accurate color reproduction of the product/clothing is a must, an ExpoDisc or WhiBal card or Color Checker chart is a good way to ensure accurate white balance. Whatever white reference you use, make sure it is illuminated by the same lighting as the subject.