The magic is to get the new valve set the proper distance behind the wall so the trim fits well.
You also want a vapor barrier behind the cbu (brand doesn't matter, but Hardie is less likely to crack when you screw it down if you get it a little deep, but is harder to cut cleanly). The vapor barrier can be plastic or roofing felt, which should end over the tiling flange of the tub. With 12" tiles, you can stop the cbu at the tiling flange, since the vast majority of the tile will be supported on the wall. Otherwise, the cbu would bow out trying to go over the lip and give you grief. If you were using a small tile, you'd have to do something else like put in shims so it would fit over the lip, or notch the studs and move the tub in so the lip was even with the studs above.
Since this is going to be used as a shower as well, DO NOT use mastic...use thinset. Industry standard calls for caulk at changes of plane (corners and the tub/wall junction).
Check out www.johnbridge.com for tiling help.