Unfortunately I don't have any more access to Word2000 or 2003, but if I do remember, there was an option in the grid options (I usually could get it from the Grid setup icon in the drawing bar, if I do remember).
Maybe the option was not specific to the ruler (I don't remember the name of the option) but it did affect the ruler behaviour.
And as I could setup the step scale, it was very easy to define default increments in the ruler.
But I agree that the factory default setup was not very user friendly.