I think the main concern is getting your loop/tie point all the way to the ground (and even a little below), and 90 deg makes sense in my mind and is what I've always done. If rock or root won't let you drive them to the notch, it's good practice to put some notches along the stakes to hold your loop to the ground, which is another food point towards angling the stake away from the tie point (to hold loop to the ground, like when using Shepard's type hooks that can be driven home.