tile installation around tub

I am about to install a new tub in my daughters house. She has already gutted the bathroom so I have a clean slate. She wants the walls done in ceramic tile in the tub/shower enclosure. I plan to put up a membrane under the cement board. My question is, where do you end the bottom of the membrane. I would think it should overlap the top lip on the tub but I'm not sure. I've always heard that you should leave a small space between the tub and the bottom of the cement board to prevent any water from wicking into the board.Pros-cons??
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