Thanks, Eddie.
I've only made about 8 squares (6 completed windows) and I've consistently forgotten to tack down the points. I haven't encountered any problems (yet), but henceforth I'm going to tack the corners down - at least until I can see what difference it makes.
BTW, it occurs to me that one could use really big squares with this pattern as long as the "window panes" were quilted in a separate step. For instance, if a window pane were 7"x7", one could secure everything by doing quilting within each pane..
For example, since I like the idea of framing something in each window, I could add quilting stitches around that framed object/picture. Of course, that quilting would show on the back side - how delightful!