I'm half way through putting up new tile and realized I did not apply mortar and tape to the inside corners of the Hardiebacker. How critical was this step? What purpose does a tape corner serve? Can simply caulking the corner remedy the situation?
Tape and thinset on the corners and flats are a critical step. The tape serves just as drywall wall taping. Keeps the corners and flats from cracking. Chances are very good that moisture will penatrate into the corner and on to your framing. Caulking the corner will still allow water to seep in from the grout line beside it. Grout and sealers are not waterproof.
Work is for those who can not fish.
Take it from someone who spent 4 months tiling a small bathroom only to have to redo it ALL immediately - that you should just remove as best as possible the area around the corners, tape and mud them, and then start over.
Not only will you risk cracking in the corners and other problems - not having a firm corner will weaken your bedding and tiles just might start to fall away from the wall.
At this present moment in time I am making cabinets for the kitchen - just in case you wanted to know what I'm doing when I'm not around.