It depends on the horse. Certain colors I would clip a week out so their color would come back in and they wouldn't be dull (buckskin, black, possibly bay). Others I might clip the day before or two days before (chestnut, grey).
It also depends on the weather. They tend to grow hair if it's cooler out so I would keep either a blanket or sheet on them until the show.
I would always clip their ears, bridlepath, and muzzle the day before the show and razor as well.
Different size blades make a difference too - I prefer using a 15 on all bodies no matter when they are clipped. And I always use a 30 on their ears, 40 or 50 on their bridlepath and muzzles.
Not sure if this helps....