I edited my video in Preview quality, then switched to High Quality for rendering. This switch makes all my 'Composite and transform' effects change position on the screen. (The X, Y, W, and H numbers all stay the same, it's just the actual image on the screen that changes. The change exists in the actual rendered video as well.)
One of the things I think is effecting this is that the base video is 202x360 (shot via portrait smartphone), and the transformations on different tracks (the clips on different tracks are all .png's of various sizes) are made in relation to that track. In Preview Quality, the transformations appear to be made in relation to the actual 202x360 video, while High Quality mode they appear to be made in relation to the full size of the screen.

Is this the only kind of error I can expect in Preview quality, or should I always try to edit in High Quality?

Strange. It should be no difference between Preview and High Quality. See here: https://kdenlive.org/en/project/timelin ... mpositing/