While the backerboard (cement fiber board) manufacturer's instruction often calls for some type of mortar/adhesive for attaching to the subfloor, many people prefer to skip that step. The recommended system of closely-spaced screws is more than adequate. You may want to tape the joints, though, and always make sure to stagger the boards so that you don't have 4 corners meeting at the same place. That will give you a weak point where tiles will crack.
Also, I hope you are not installing "porcelain" tiles on any floor, especially in the kitchen where there is heavy traffic and frequent spills.
Also, I hope you are not installing "porcelain" tiles on any floor, especially in the kitchen where there is heavy traffic and frequent spills.
