or can i leave the seams alone? if i have to tape them what tape do i need and what compound do i need? thanks for any help!
There's no need to tape the seams of cement board, as long as you get a good close fit it should be ok. You've made a wise choice using cement board, at least on the lower half of the shower enclosure area, greenboard, while being somewhat moisture resistant is not impervious to water damage. It should be used above the cement board and will do a good job there. The seams in it will need to be taped, at least above the top edge of the surround.
No need to tape the seams with a shower surround. in fact, I would just use green board instead of cement board. Cement board is mostly used for tile and the joints are sealed with a fiberglass mesh tape and tile mortar.
