ever you start the tile, the cementious board must be there. The tile is afixing to the cementious board. So, if your tiling the tub lip down to the base of the tub, then the cement board should be over the lip and where the tile begins.
I personally like to tile over the lip, but NOT have the tile touching the base of the tub. This is where white silicon caulking would come in to play. The space might be only 1/8 inch.