Last OI I built out of 3/4 Redwood I used none in the bow and I think one on the stern. If your strips are bending around and up easily, don't worry about it. Since the back is low in depth, you may need one since you won't have enough strips on the sides of the boat to fill the distance it travels up to the tip of the stern - you'll automatically see this hapenning as you work along.

so, don't plan for the worse if it's not happening.