Shizzy, my husband had 2 EMGs in early 2008 and again in Sept 2009. We changed doctors early part of this year so the new doctor wanted his own EMG. I don't know how the '08 ones compared to '09 but the EMG done recently was more extensive. Earlier EMGs were done on his legs only.
I think it must be very individual by doctor, the practice I go to hates to order EMGs. That bothered me after my progression sped up crazily, but I came to peace with it. What it is and the course it takes matters a lot more than a name!
I was diagnosed with bulbar PLS in 2006. In 2008 I noticed my limbs being affected. I didn"t have another EMG until 2009, which showed LMN damage. My diagnosed has been re-classified now to ALS.
So I would think that when new areas start being affected, or you start seeing marked advancement, request another EMG.