Yes, you need to countersink them. Note the reference that I made for the step bits the Jantz carries. That's what they do. Others use different sizes of regular bits but I find the step bits made for the job to be the easiest way to go. To apply the bolts I glue a scale to one side of the drilled tang, drill the scale from the tang side after the glue dries. Then I glue the other scale on and drill it after the glue dries. The bit used to drill the hole in the tang and the scales should be the same size as the narrow part of the step bit. I then use that hole as a guide for the step bit to countersink for the head of the bolt. Make sure that you have drilled deeply enough for the halves to screw together. I like to put a dab of epoxy to the threads of the bolt as I assemble it. Once they are installed you grind them down to the shape of the handle just like you would for pins.
Doug