Well it turns out Witch is in season, so it's safer not to bother. This is her first show and she will be otherwise turned out beautifully [apart from her tail and a conspicuous lack of hoof black, it turns out I haven't got any so I'm screwed unless someone will let me nick some of theirs]. We're not going for the placings and I plan to apologize to the judge for our presentation regardless, but I doubt we'll be the only ones with an unplaited tail... and an unplaited tail looks WAY better than a bad braid which is very common at this show. It's only a local thing. There's usually someone who's presented way better than anyone else and they tend to take all the ribbons but I'm not going for ribbons, I'm going so my horse gets exposure.
But, I will take all the tips in this thread and use them for the next show :)