It is my understanding and belief from my research on the issue (I wondered about it as well) that if you have any reason to believe that EMI from your computer is screwing up your monitor's display, speaker sounds, etc. you should turn it on or experiment with % setting if that option is available to you (Most computers just say "on" or "off"). If there is no EMI issue with displays, speakers, nearby radios, etc. from your computer, then turn it off.