Plain classical rollers rotate plastic to wood inside the headstock.  It is recommended to add a dry lubricant inside the inner hole where the roller located to prevent squeaking and allow for a smoother action.  Bar hand soap or better yet, candle wax works well for this purpose.  Please consider using rollers with bearing bushes instead. More information here.

A clearance hole in the headstock should be used behind the side-plate.  This is to keep the roller properly rotating with its bearing diameter in the side-plate.  This is important because the accuracy and performance of the gears is set by the exact position of the string roller.  It also prevents squeaking of the roller on the wood.