Back in the time of Dinosaurs, Ol' Nick and I went through the US Army Aviation School where they told us we were helicopter pilots. At that time we all learned everything about the "Dead Man's Curve" commonly known to civilian pilots as the Height Velocity Curve or H/V to Bell drivers. We were well versed in flying "Low Level" or by means of a method called "Contour Flying". Both methods were designed to allow us to avoid the dreaded "Dead Man curve" even if it prevented us from being able to get to a safe forced landing area if we had an engine failure. (think single engine aircraft here).
While we were away on our Senior Trip....we were told to forget about Low Level and Contour flying and start flying at heights of 1500' or more and to avoid being repetitious and committing sins like following terrain features and roads and such.
After that period of unpleasantness between our employer and some Southeast Asian gentlemen, our employers took on some old business with folks who liked Bears. It seemed the Bear Boys had lots of tanks, mobile anti-aircraft guns and the like and seemed very willing and capable of using them.
Our employer realizing the old SEA method of flying at specified heights above the ground for administrative and seperation of aircraft types would not work well in the now rather hostile threat enviroment.
We then were told to fly "Nap of the Earth" known by the working class as NOE. There we learned to vary our airspeed, speed across the ground and height from obstacles and such as required to remain out of sight like our bosses did whenever there was work to be done. Amazingly enough....there was no more mention of the H/V curve or Dead Man's Curve lest one be flogged for Heresy and Back Sliding.
Thus....assuming what Ol' Nick and I were taught there is either no safe height to fly or no unsafe height to fly....but one thing is certain....bend your birdie and you will be wrong.