250' AGL, 40kt and 80% power in a climb is way different than 250' AGL, 40kt and 40% power in a descent. In the first case, the RRPM will drop much more, and it will take longer for the autorotative airflow to develop. Given the choice, I'd much rather be inside the curve in a low-power descent than a little outside the curve in a high-power climb.
In fact, it would be very difficult to make normal approaches and stay out of the H/V curve. IMHO, the only time you truely enter the curve in approach is on a steep approach, where you are operating at a relatively high power setting while still pretty far from the ground.