Whatever you do, do it very early in your trip. If you are spending a week in Disney, you won't enjoy yourself at all if you keep putting it off till later in your trip. You will be nervous, only thinking about the proposal, and not able to really relax and have fun.
I proposed to my wife at Cinderella's Royal Table. They treat you so well in there, always calling your girlfriend "Princess" and you "My Lord," or something like that. Anyway, Between dinner and dessert, i pulled the ring out of my pocket, and asked her if she would be MY princess. The waiter happened to be walking toward us, so he picked up our camera from the table, and started taking pics. Needless to say, he got a huge tip.