For open soffits, however, we cut the rafters plumb with the plate line and add 2x6 tails flush with the top flange of each I-joist rafter.
We install the squash blocks on each side of the rafter — one below the 2x6 tail and one on the opposite side.
We cut eaves blocking from I-joist stock. Whenever possible, we like to install it as we go when setting the rafters, just like we do when setting trusses.
Having to cut a birdsmouth complicates things, and adds much more time than it takes for us to rip beveled strips on a table saw.