The rule of thumb is as follows:  If when you drag your finger across the
rotor or inner-drum surface and your fingernail catches in the
grooves...you should check the specs to see if a grind would be feasible.
I used to work for an auto-parts shop that routinely ground rotors that
were obviously fine; yet another gimick for quick cash.  A little research
and a micrometer can go a long way toward avoiding this pitfall...

Seth B.
'84 Westy