From what i gather from the stables up the road they work with the pony's shape, like they took a colt to his first show, but they told me his neck wasn't thick enough so they left his mane longer but thinned it out and made it look neat. They used a leather show bridle on him.
Clipping is not required.
I have also seen them pulling manes of some of the ponies and trimming the tails to fetlock level, also giving them a bridle path. i think it entirely depends on your area.
(someone correct me if i'm wrong, i'd hate to be giving out unhelpful info! )