If it were my bathroom, I'd use the blueboard throughout the whole bathroom. As far at the tile is concerned, I'm not the expert here, but I'm currently working on a project that involves several other craftsmen. The bathroom has the tile backer everywhere that there will be tile. I also have worked in several bathroom where tile was added right over the drywall instead of using backer. All of those bathrooms had problems with tiles falling off and with the drywall disintigrating.
Others here may differ, but my advice would be to make up your mind ahead of time as to wether your going to use tile or a surround and if tile is the answer, then definatly use the backer.