Freewheeling diode on regulator output
Assuming you are talking about a topology as can be seen below:







simulate this circuit &ndash; Schematic created using CircuitLab

Then yes, there are valid concerns to at least consider doing this with (some) linear regulators (in some situations). 

In the event that the input is forced low &amp; especially if there is larger output capacitance than the local input capacitance, you end up in a case where the output voltage is higher than the input with a relatively high source of charge. 

This is a valid concern in the automotive industry and you find regulators with such a diode on-die. 


  SHORTING THE REGULATOR INPUT
  
  When using large capacitors at the output of these regulators, a
  protection diode connected input to output may be required if the
  input is shorted to ground. Without the protection diode, an input
  short will cause the input to rapidly approach ground potential, while
  the output remains near the initial VOUT because of the stored charge
  in the large output capacitor. 
  
  The capacitor will then discharge through a large internal input to
  output diode and parasitic transistors. If the energy released by the
  capacitor is large enough, this diode, low current metal and the
  regulator will be destroyed. The fast diode in Figure 21 will shunt
  most of the capacitors discharge current around the regulator.
  Generally no protection diode is required for values of output
  capacitance ≤ 10 μF.


http://www.ti.com/lit/ds/symlink/lm340-n.pdf