And of course the diode current rating must be sufficient for the relay current. So I would not use the 1N4148 or 1N914 small signal diodes as they are rated at only about 100 to 200mA.
Also, small signal diodes tend to fail open, whereas higher current diodes tend to fail short. In one case, it potentially destroys the driver transistor by presenting an overcurrent condition, in the other it may put a big voltage spike into the Vcc line and destroy other components. So I avoid using small signal diodes as flyback diodes.