its your project- do it with the technique you prefer.
cutting out the excess fusable cuts down on the stiffness- and trimming cuts down on bulk- but it is not necessary if you would rather not =
i pretty much never (window) my fusable- i cover the whole piece- i like it that way-
i trim overlapped pieces to a size i am comfortable with- i don't measure- i cut it out making sure some will be under the piece it's supposed to be without fraying-
there are no hard rules- you can use any technique you want on your project....often we love a pattern- buy it- read it- and decide- well i can do that===this way instead of that way--- nothing wrong with that.