Today was a bit of an eye-crosser. After looking at Chapter 9, I realized I really needed to get in there and make more text boxes like a proper book designer, so I went back through the previous eight chapters and fixed anything that really wasn't satisfactory.
I believe the source of the problem is the amount of space between paragraphs--I used an odd number of points in there, and I think that's why the two sides of the spread are sometimes off by half a line. (A line off would be simple to fix, because you can usually figure out how to put in or take out a new line, but half-a-line off means it's just never right.) Live and learn--hopefully next time I'll start with a more-efficient template.
Anyway, then I went and lined up the spreads for chapters 9-11. Then I realized I hadn't actually proofed chapter 9-11, so I did that. Mostly what I'm catching there are extra spaces I put after em dashes to make a line break properly in the original edition, but I also caught a typo that had made its way into both the paper and e-book edition!
I should note that because this is the large-print edition, I'm always lining up spreads by inserting more space into the shorter spread--I think that serves the purpose of a large-print edition better than crowding up the text lines in the long spread.