Since I designed this house I had a lot of leeway as far as moving a wall and inch or two if something wasn't gonna fit very well but that's a last resort kind of thing. If you're going to use standard sized tubs or showers you need to remember to set your wall back far enough so the finished
dimension is what you need. By this I mean don't make the mistake of framing in for a 60 inch tub then finding only 59 inches after you sheet-rocked the room kind of thing and remember if you use half inch backer board and tile you also need to allow for the width of the tile and mortar.
I went as far as making the exact finished dimensions in a manner that minimized the amount of tiles I would need to cut. This might not be that big a deal if you're just doing one room but I did all my bathrooms, hallways, entranceways and the kitchen floor in tile so I had to put down about 1700 square feet of it. A little planning ahead goes a long way if you're just in the designing phase...
I used backer board for the walls of the masterbath and the heated part of the floor but in the rest of the house I did mudpans which if I remember I'll do another Instructable on and link HERE. (http://www.instructables.com/id/An-Alternative-to-Using-Backer-Board-over-Subfloor/
I chose not to heat the base of the shower itself because I didn't see anyway a low current system could keep up with water flowing over it's surface and I had to line the subfloor with a sheet of shower pan and didn't want to pierce it with the cable guides and I was also worried about running the heating coils up against the rubber. I didn't see much point in heating the floor under the vanity either so I chose the size of the heating strip accordingly, which was about 45 square feet.