It is true that the contours will change over time with water over them. Trenches will become shallower and hard breaks will become softer. Hard clay resists this better than softer soils, but will occur some. With this in mind you should dig deeper and wider depending on your soil. One thing to remember though, a contour is a contour to a fish. If the alternative is a smooth bowl for a pond bottom, a one foot silted in ditch or ridge will attract a lot of fish. I like to work with contours and hard cover in conjunction, not just one or the other.
-HH