not totally necessary but if its never been done you might consider it. not exactly like wp while doing a timing belt because the water pump replacement adds usually very little labor cost to a timing belt job. with wheel bearings you do have to tear into the hub a bit farther than just doing an axle, i would say ask your mechanic what the labor if you did it with the axle, then ask how much it would be if you did it later by itself, then compare the prices.