No, it wouldn't. Since upvotes aren't public it's useless to upvote something to show support of a subthread that is already on top in its thread.
That's true, but if more people did this, threads would be unbearable to read so I think it's better that we agree not to. Highly voted posts will appear closer to the top, so upvotes are not entirely wasted.
I second that.
† Some people just don't have any sense of humor whatsoever.