Ideally it should be evenly distributed through the whole grip, as long as you can feel the blood being pushed toward the head you're doing it right, your head will 'plump up' when completing the stroke. Just play around with your grip until you're satisfied you are accomplishing this.
The grip you are referring to is the underhand grip, thumb on top & palm facing upwards, you can flip it the other way too with the palm facing down so the thumb is on the underside, try either and go with what feels best.