I'm can tell you that damage to the car by valets does happen. It happened to us. Not at WDW, but at a very well-known hotel in Hawaii. When the valet brought the rental van to us, it had a big dent that extended all the way along the driver's side of the van. It was clear that one of the valets had hit it against a post or something and just kept going! The problem that arises is that it is your word against the valet's. And, of course the valets are going to claim that the damage was there when the car was dropped off. Fortunately, we had taken the collision damage waiver, so we had no issues when we returned the van to the rental agency.
Since that time we've been very leary of valet parking. Also, I hate the thought of damage caused by a valet becoming a claim on my auto insurance policy, with possibilities for premium increases, surcharges, etc. And, Disney may have plenty of insurance to cover damage by their valets, but try proving the valets did it!