I agree with countercanter.
Overall, you want to look well put-together and you want your horse to look the same! Personally, I would clip (at least a trace), pull the mane (or touch it up), and likely braid depending on the clinician. Freshly trim and, if you do this already, pull the top of the tail as well (I love a good pulled tail!). You may not be able to bathe due to weather, so get there early and give a really, really good grooming! If your horse has any white markings, make sure they're sparkly and clean!
"Always be yourself. Unless you can be a unicorn. Then, always be a unicorn."