Here's what I was always told and it makes good sense and works for me. The quickest most direct strike is in straight line. We represent this by shomen uchi. So shomen uchi is always uke's perferred choice. But if nage has a good kamae you cannot get in for shomen because their guard is in your face (running onto the sword). So in this case, uke steps around the guard and performs shomen from the new angle - which ends up striking the side of the head. This method also stops the big loose swinging strike - IOW it's a slightly more difficult problem to solve.
That's why it sometimes annoys me to watch people without their kamae up receiving yokomen - I can't help but think why is uke taking all that extra effort to move to the side when they could move straight in and strike.
Having said all fot that, Mark alludes to a reasonable point. Our attacks are, as we are so often told, representative. Designed to teach us different angles rather than mimicing exactly any sort of attack we'll find "out there". The other method you describe may well be useful to simulate certain angles of attack (step up hook?). But the strikes come the sword so by and large should follow sword based logic.