If you wash your jeans too soon, you run the risk of ruining your fade, as well as any future fading (premature washing will eventually make it twice as hard to get that perfect creasing). Soaking the jeans in warm to hot water for a couple of hours will shrink the jeans to their natural state, so you have a better idea of where the real hem length will fall. It should also remove some of the excess indigo dye so you're not leaving blue stains everywhere you go.

Remember that the hotter the water, the more they will shrink.

For more explicit directions, check out Rawr Denim's soaking guide, or Hall of Fade's initial soak instructions.

Quick note: If the denim is sanforized, it's not necessary to soak the jeans. Just start wearing them.