Put my vote down for balancing.  I have had multiple times where I just picked up the car from the shop (who used a Hunter machine) and there was still a shimmy at 70mph.  I think if you sneeze on the wheels it throws them out of balance so I suggest taking the car back and getting them balanced until they get it right. 
 
 
BTW-Both times it was on 18" wheels both OEM and aftermarket on my 2002 S.