It should also be noted that you don't have to run a 'zero' deck. Depending on your build, having a deck clearance can be beneficial, such as in a forced induction (turbo, supercharger, etc.) project. It can be used to square a block the was slightly miscast, or in the case of aluminum, warped.

In either case, it is good to have this done to make sure all surfaces are square to the crankshaft. It will helpkeep the volumn of the cylinders consistant from cylinder to cylinder, much like having the head surfaced and cc'd.

Keep in mind that if having the block decked is to increase the compression like the example above, this can change the way the intake mounts to the heads (in a 'V' configuiration) and may require to intake also be machined. In the case of an inline engine, this can cause issues with OHC timing belts and chains because you will have decreased the distance between the crank sprocket and cam sproket.