Do I use grout, or just abut them togther?

I am installing slate around my gas fireplace opening, about a 6" border all the way around.  Do I need backer board, or is 5/8" plywood sufficient? My largest span between studs is < 8".  Do I use grout, or just abut them togther without?  A salesman told me this is how to install slate because the grout would stain the surface if I used it.  Finally, he said to use mortar with latex, not an adhesive necessarily.  Any advice??