Just a word of caution... if you are not staying at the resort that you are valet parking at, it will be cash only. While staying at Pop Century, I had dinner reservations at Artist Point, and we drove. (We only drove because it was the night before Hurricane Jeanne, and parks were shutting down...)
When I came to claim my car, I handed the bell captain my room key, and he told me that it was cash only. Only WL guests could charge to thier room. Thankfully I had enough cash on me, but what if I hadn't? That would have been a scene.
As for tipping, one of my old managers used to be a valet manager at the GF (she is no longer employed by the company) and she told me that it was best to always tip at drop off as well, because they were the ones that could do the most damage to the car. Not that anyone at Disney would do that, they would never, but, it would increase the chance of the car being parked in a more desireable parking space, rather than below the tree where the pooping seagulls sit.
(Of course this only applies to if a valet provides a worthy service, and takes your car... if you have to stand there and wait, and then approach the bell captain yourself, interrupt the conversation about last weekend, and remind them that they have a guest who needs a car parked, then a tip is obviously not neccessary. Not that this happened at the WL for me. No, not at all.)