Apply grout after the mastic has had a day to cure; use sanded grout for gaps of 1/8 in.
or more and unsanded grout for narrower gaps.
The one spot where caulk, not grout, should be used is the seam between the countertop and the backsplash.
The right material for that is a top-quality silicone caulk.
How do you Recaulk a kitchen backsplash?
How do you seal a countertop to a wall?
Where does backsplash meet countertop?
Typically the tile backsplash stretches from the countertop to directly beneath the cabinets, although some may be only 4 inches in height and some may sit on top of a 4-inch granite backsplash. The material and size of the backsplash tile has no effect on how it meets the granite.