A couple of people have asked me if I did end up trimming my blocks before adding borders - no, I didn't. I'd decided to just wing it. :)
And of course, when I tried to have the 2½ block inset border come out even at the corners, I had problems. What I ended up doing was pinning it on, seeing how much too long it was to come out even, unpinning it, taking an extra-wide seam in two or three places, repinning it, then sewing the final seam. Then it was easy enough to just do the same thing on the other side.
I did want the quilt just a little bit wider (can you tell I'd added an extra row of strip twist block quarters along the bottom?) That made it plenty long, but I chose to sneak in wider side borders and narrower end borders.
I don't think you can tell which of my 2½ square blocks are really not square, unless you really start looking at them with a critical eye. Or can you? :)