As far as QT goes, to be on the safe side I would think the same procedures as a new acquisition would apply. Exposure to "foreign" fish is no different than a new purchase as far as I'm concerned.
Koi disposition would be a go/no go decision maker for me in showing. A fish with a bad reaction to handling is likely to fall apart (bloody gills, fading color, etc...) and not be in show condition anyway, but moving it from the pond to quarantine/purge tank before the show will tell you how it reacts. If it can't handle that, don't take it to the show. I suppose that part really comes down to what kind of "relationship" you are able to establish with the ones you plan to show. If they aren't comfortable being handled by you at home chances are slim they'll fare well under show conditions.
Bear in mind, this advice is coming from a guy who has only transported Koi a few times, and never to a show...yet
