The vertical wall seams crack more readily because the house shifts and moves. Ive tried grouting only and going back to caulk with sanded grout caulk or silicon only after a crack develops, hoping in some cases it will not. Sometimes it does not crack. Some contractors caulk all seams as a matter of course, no callbacks, safer. I liked the idea of sanded grout caulk to match your grout color It has not held up for me in wet conditions, such as the top of the bathtub where the wall tile starts, etc. , definitely not along the shower floor where the wall tile starts. It has held up in other areas though, and looks nicer when it all ties in. I agree, mildew on silicon is unsightly. Every bathroom has different conditions, including ventilation, moisture, etc. Some people have to clean out and recaulk the silicon every year or two. Whether its allegedly mildewproof silicon or not.

once you install the cement board (so that it sits above the tub flange), fill in that channel with thinset, and let it harden. once hard, go ahead and install your Kerdi, so that it's sitting no more than 1/8" off the tub. Then you can seal it with Kerdi-fix. That's the "book" explanation. If it were me, I'd use the same color siliconized latex caulking as my grout and seal it with that. Reason being that then you wouldn't have to worry about the color of the Kerdi-fix sticking out from under the tile.

cwalen, we're in a converted industrial warehouse building with concrete floors, so we didn't have much choice --- we had to build the floor up a little so the drain (we used a linear drain, which needs less buildup than a regular drain) wouldn't stick up. Then we built a curb which is tiled. We have not done a door at all yet -- just a shower curtain. I like the curtain, it looks great and it's easy to clean. No leaking as long as the curtain is closed. I don't seem to have a decent picture on my computer, but if you need pix, I'd be happy to take some tonight.

Plumbing system would also be different as you would need a diverter plus fixtures for a bath and they would think they would all have to work together. I did the opposite (removed tub/shower for a larger shower) and my drain also had to be changed. A bath generally has something that stops water to fill up the bath whereas a shower drains continually.