Yes, you do have to include the width of the sashing strips when you figure the size of the setting triangle. If you don't you will get this weird looking yellow triangle where the block sticks over into the triangles.
If you do, the setting triangle neatly matches the end of the block.
As I was cutting the triangles, the book I was using (which I won't name) didn't mention how sashing would affect triangle size, so although I THOUGHT the triangle should be bigger, I managed to convince myself that I had done this before, kept the size the same, and it worked. It didn't.
Then I looked in Setting Solutions by Sharyn Craig, and there it is: a clear statement that you must include the sashing width, therefore cutting a bigger triangle. She has a neat little chart, telling you what size to cut, so there's no math required.
I was in denial for a while, thinking that the weird look would go when I squared up, trimmed the excess and added a border, but I finally had to face the truth, rip those triangle out, and sew in bigger ones.
Actually, this isn't as bad as it sounds. I have postponed making a decision on the next step. Still, it was pretty much a wasted day. So be warned, everyone.