I tried that too for CWU and it worked quite well.

What I can't figure out is -after having used the method- I didn't use it when I started on the Maiden.

Denny said that he doesn't recommend that method because it can cause problems with a good seal between the two layers of the wall itself, which is "more important" than the window.

I've been meaning to experiment with it myself, but never quite got the time.

I actually wind up with a better seal. Since I use glue fairly sparingly, there do exists some "dry spots" in any sandwich-fold pieces I do. When I glue-then-cut-then-glue-windows, sometimes I've had bits of wall around the window come apart. Doing it beforehand, I get a much tighter seal.