It's good to see that we both approached the problem in (almost) the same way :)
If the diode is on the 5V side of the regulator, it decreases the 5V by the diode's forward voltage, whereas if it's on the 12V input side, it doesn't affect the 5V regulated output. But, of course, the diode we both put in the USB 5V line also decreases the 5V in that same way. One advantage of having the diode on the 7805 output is that it avoids the small current drain (just a few mA) through the regulator from 5V to ground when it is not connected to 12V input. So I guess from a practical point of view it does not matter if the diode is on the input or output side of the regulator - both versions solve the problem!
If the diode is on the 5V side of the regulator, it decreases the 5V by the diode's forward voltage, whereas if it's on the 12V input side, it doesn't affect the 5V regulated output. But, of course, the diode we both put in the USB 5V line also decreases the 5V in that same way. One advantage of having the diode on the 7805 output is that it avoids the small current drain (just a few mA) through the regulator from 5V to ground when it is not connected to 12V input. So I guess from a practical point of view it does not matter if the diode is on the input or output side of the regulator - both versions solve the problem!