They need to be consistent within the context of how the strike zone is defined by the rules, how whichever sanctioning body interprets this definition and wants it enforced and, to some degree, with whatever expectations there are for a given age group or skill level.
If the umpire is always calling strikes on pitches a foot outside, over the batter's head or on pitches that bounce off the plate he is being consistent- consistently bad!
There are pitches that go through the huge strike zone in softball that even your grandmother in the cheap seats can tell is a strike. The umpire should be calling those all the time.
There are pitches that are so far out of the strike zone that they are obviously balls. Those should never be called strikes.
And then there are those pitches that skirt the defined edges of the strike zone. These are the ones where the umpire's judgment and consistency is put to the test.
Before he can get to the point where he can refine his judgment and consistency on those pitches, an umpire needs to: 1) Have a solid grasp on how the strike zone is interpreted, and; 2) Be set up behind the plate in such a way that he can actually see the entire zone and the plate.
If he never gets a handle on numbers one and two, he will never make it successfully to number three- being consistent on the borderline pitches.