Well both are possible. When used whole, it tends to be when you intend to remove them later, like if you were to use whole lumps of cinnamon bark. It is mainly used in powdered form. If you just crush them, then you get tough bits that can feel unpleasant in the mouth. Freshly powdered coriander seeds are much more fragrant that if left for any time. You can powder it with a pepper mill, though as it tends to be used by the dessert spoon full some powered appliance is less boring.

It is a common component of curry powder.