Not normally, but it should be if the steering rack or pump are ever replaced. Under normal circumstances, the fluid in the power steering system should last the life of the vehicle (or the life of the major power steering components, whichever comes first). But as the system accumulates miles, microscopic particles of metal and rubber can buildup in the fluid. These particles can act like an abrasive and accelerate pump and gear wear, so the fluid should be changed if the original pump or rack has failed to prevent contaminating the new parts with dirty fluid.
Even though the fluid in your power steering system does not normally require changing, it's a good idea to check the fluid level periodically (say once a month or when changing the engine oil and filter).
If the level is low, add fluid as needed to bring it up to the full level (hot or cold). Then inspect the hoses, pump and steering gear for leaks. More than a few ounces of fluid in the rubber bellows of a power steering rack indicates internal wear and leakage.
Always use the type of fluid specified by the vehicle manufacturer (Dexron II or a special blend of power steering fluid).