The water pump should be changed with every timing belt change (or so I have been told by just about everybody). Apparently the tension of a new belt on the bearings of an old pump will wear out the pump in a short amount of time thereafter. Since you already have most the work out of the way when you get the timing belt off, replacing the water pump isn't really a big deal. Also, if you have no history of the car, I wouldn't chance not changing out the belt, just for safe keeping. In the end though, its your call. Hope I helped.

Peace--Air