Important side note that really doesn't get enough attention: Make sure you rinse ALL the suds of soap OUT of the hair.
... and don't squirt the horse in the face with the hose.
If it's kind of chill in the air... bring a bucket of warm water and a spundge to comfort them and keep them out of direct colder winds.
(kind of sad when they shiver)
If you don't plan on rideing them or working with them after, say like after a hard ride you want to bathe them dry most of the water and throw a blanket on them.
If you turn them loose after ... expect them to roll in the dirt. (@#$%)