I guess all of the hardcopy books I read had shadows, gradients etc from the external lighting. I just lived with it. Same thing here: sometimes I see blotches on my Voyage. Sometimes I don't. Bottom line is I guess I don't expect the screen to be entirely smooth. Especially if we want to get closer (as the Amazon people say) to a "paper experience".
I have the Omoton cover, which is light and simple. It makes the kindle more like a book. I like the feeling of it rather than the hard plastic of the device. Also somehow it manages to stay closed in my purse, which is great.