Are there plugs for unused fin slots? I cannot find any, and with all the attention to shape... I dont see where 2 or 4 rectangle 'holes' being irrelevant to performance... I could be wrong, but it would seem to have a significant amout of drag and/or turbulance.
Once the plug holes are filled with water, they create very little drag. Fin slots create a bit more, but not much to worry about. Your actual fins create MUCH more drag and turbulence. Keep in mind the plug holes are also oriented in the direction of flow most of the time.