I don't think it has to be spotless. I think they just don't want chunks of gunk in the bottom of things. I do the "water stretch" Wulfie suggested with just about all liquid cleaning and personal products.
It also helps if you soak the inside of the bottles. Put a couple tablespoons of water in, swish it around, then turn the closed bottles upside down over night, pour out in the morning. Good enough.