Chimbu -
You can run LOP without the EDM - that's why the IO-550 flight manual tells you to do it. In fact you probably have a greater chance of damaging the engine (burning valves etc) if you are not aggressive enough with LOP running since your EGT will be higher.
I agree, however, that the EDM removes all of the guesswork from the equation.
Running at 'moderate' power settings almost eliminates the possibilty of 'overleaning' most engines (i.e. they are happier really lean than over rich) - particulary with regard to worrying about LOP operations. However, I would always urge adherance to the manufacturers handbook for your motor if only to preserve the warranty.