If affiliated to breed society DEFINITELY plait (I know this for sure) if unaffialated I would probably leave unplaited (of that I am not certain - opinion only) - Good luck
in part bred classes you turnout to type ie if it a hunter type then you turn out as though it were a hunter class, if it is a hack type then you turnout as a hack, showpony type as a show pony and a show cob would be shown hogged and trimmed.
The only horses shown unplaited are the arabs and the purebred M&M's
Hell, I'm never going to get our youngster to a show then (3/4 Sec D) as I've never plaited a mane that's stayed in for more than five minutes on a placid horse, let alone a little firecracker!