I have installed hardiback flush with the shower pan...am planning on using thinset to mount tile on walls. question is do I put thinset all the way from hardibacker to bottom of edge of pan or just thinset the backerboard and caulk in the space between tile and edge of pan? I'm concerned about the airspace from edge of pan to where the thinset starts on wall...thanks.
You MUST caulk between the last tile and the shower pan simply because the shower pan will flex while the tile wall won't. Grout and thinset will surely crack or at the very least, separate away from the shower pan over time. Be sure to use a very hi-quality siliconized acrylic caulk that is designed for use in wet environments AND specifically touts two features among others: high adhesion and flexibility. While everyone likes to think that the shower pan is an immovable object, it will flex by its very nature.