Was it coated heads? If it didn't wipe anything away immediately, its probably gonna be fine. If uncoated head, should be just fine. Personally, I would make sure its all of, by gently wiping with damp cloth. (dampened with water and a lil soap, then wipe again w just water n cloth, then dry cloth.).
play it for a while. if it sounds bad or effects the head change it. Don't think about it too much, just remember if the head does something weird, that is why. I think the worst will be the smell for a few days, as long as it didn't get underneath into the drum. WD40 is just a water displacement. It's main use is for cleaning (not lube like everyone thinks and uses it for). You're going to be just fine.