A full silver C isnt really necessary. A bronze perhaps or even just solo gliding experience should be enough knowledge to know your way around as a tuggie. We have both types(PPL's with gliding experience and those without) at our club and to be honest the guys without experience dont have a clue where theyre going. We're a small club though and its not a huge problem, you just have to constantly steer them a course on the radio and tell them where to drop you. The guys with experience just blast off and know exactly where to position you. They know about wave, they know how thermals work etc