Very true.
Higher is better, but most effective upto the point to bring antenna above all obstructions, so that it can “see” the horizon all around. Any further increase in height increases the cost & effort, with little benefit (unless the increase in height is in hunreds of feet).
Since the maximum POSSIBLE range at any location depends on terrain, curvature of earth and antenna height, and aeroplane height, it is important to determine this for your location. Please visit the thread shown below for step-by-step guide to determine your maximum range. Generally the maximum flight altitude for commercial planes is 40,000 feet, I normally use this figure to get my location’s maximum range.