Most good cams come ground as near perfect as possible. The reason for having different slots in the crank gear, is to make very small changes in cam timing if the block has shifted some during its life and things aren't quite as they should be.
They are also used to get more or less torque at lower rpm by change the "timing" of the cam rotation as its aligned to the crank.
Most times, using the "0" keyway is fine. You can check the cam timing with a degree wheel and a dial indicator, but for a street engine, the cams are usually fine installed at "0".