Tim and Lester's last posts are very instructive. I also have used the term "frequency" of contour when speaking of routing holes across contoured terrain.
I think that one of the aspects that have Charlie wondering if their is some 'rule of thumb' or if there is a 'common sense' or 'cents' aspect to this notion of use of contour in the parallel, diagonal or perpendicular approach is that if one only looks at tracts of land on the contour map (preferably 2ft ele for routing) one does not get the proper sense of scale and proportion at an on-the-ground eye level proportion. One must have the contour map in hand while one traverses the actual land to 'see' the effect of the frequency or severity of the elevation changes, whether on diagonal, perp or parallel aspect to see the actual potential distances the ball flight will travel and what ground effect it will have in an LZ, etc. Then, one needs to see where the best place to fit the next hole and so on down the route is best directed. Sometimes, merely a contour map and exercise in routing that a 'design contest' might entail will distort or not give accurate reflection of what a hole would really be like, on the ground. So, all the rules of thumb are not complete until the contour is matched to the actual process of routing and pounding the stakes in the ground for tees, TP LZs and green sites, where you match your conceptual notions to reality.
Also, when evaluating the contour frequency, it seems to me that one really must be very aware of the flow of the elevation grade of the low troughs in the whole property, observing the highest low point to the lowest low point, and how that can all work together to drain the rest of the elevation where they holes are routed.