It is generally a judgment call based upon the condition of the flooring. You can attach the backer-board to the hardwood flooring, but if the floor is so warped and awful that you are asking about covering it with tile, you might want to tear the hardwood out, re-lay plywood (if it is not there, already, and put the backer board over that.

You need as flat a surface as possible for tile: otherwise, the tiles will crack and chip over time. Backerboard can help eliminate some floor unevenness, but not all of it.