Actually it’s usually an easier call than that. Most images of people are framed with more dead-space on the bottom than on the top. If it’s a full-body shot, you can cover the legs or the face (legs). If it’s a torso shot, you cover the chest or the forehead (chest). From a general scene composition point-of-view, I understand why the bottom is almost always the preferred location.
This discussion is really just a matter of how to make reading text as easy as possible. If you read text at a single location that disappears and reappears, that’s one thing. But if the text shifts to another location from time to time, not only is it a little jarring, but you also lose some reading time while your brain and eyes make the shift to reading at the new location.
This is made even worse with the burnt-in subs, because your brain sees the burnt-in subtitles at more-or-less the same location as your regular subtitles, and starts to process them BEFORE saying “wait a sec, these aren’t in the right language” and THEN you shift to reading the subs at the new location. Lots of jarring/loss of reading time in that process. Which is also why I think it’s worse with subtitles using the Latin alphabet–you actually have to start reading the text to see that the language has changed! Again, in an ideal world we’d have multi-angle scenes with localized burnt-in subs and so on, but as long as we’re working around them, I want to work around them in a way that’s easiest on the reader. So that sometimes involves shifting the subtitles to a new location and leaving them there until you don’t need subtitles at that alternate location for a long time.
EDIT: And dangit, I just talked myself out of any changes to the adjacent alien subtitles at all. The reason is kids. Subtitles for Project Threepio tend to show onscreen a little longer than your average subtitle, and that’s intentional. It’s because I want kids to be able to follow along, and they read slower. Which also means all those cases that were borderline for an adult aren’t borderline anymore. So thanks for the fire drill everyone! 😕 But I will still be changing the “A long time ago…” and “STAR WARS” subtitles back to the bottom.