Yes, in general you find the best high quality source you can get your hands on and then compress that. For us lay people, that would currently be any 4k videos with a high bitrate. In such cases, it doesn't matter much that it is already compressed with AVC or HEVC. Sure, when you compress that again at a lower bitrate, there will some loss of data or quality. But honestly, it doesn't make a discernable difference (after all, you decide what is the video quality acceptable to you by choosing how much more to compress). Ideally, if DVD and Blu-Rays lasted long, we would all just be saving our videos on it. (Assuming there will be any Blu-Ray readers, 10+ years down the lane).