I dont razor my appies, I think it gives them a bit too pink of a nose, just like I dont like to razor a pinto with a lot of white on its muzzle. Just a personal preference and it has never seemed to hurt any of my horses in the show ring.
It really depends on the horse, their color, and how much mottling they have. And, how much you want to bring out the Appy. Got a pic of the horse's head?
I razored mine the few times I did show, but they did not have mottling on their heads. Melinda (Maple Hollow) does a fantastic job on her pintaloosa stallion. I think she has a close up head shot of him on her website too from a show that you could look at.