Actually, using as small a bolt as necessary for the tension strength is a good thing. You want a weak shear strength so that, should you hit on a wing tip or cartwheel on landing (thinking of the trainer planes here with beginning pilots) the bolts will shear and do minimum structural damage to the plane or wing. When using one or two dowels at the front and one or two nylon bolts at the rear, 6-32 nylon bolts are an excellent choice to hold the wing on. They will never break under flight loads yet can shear under side loads when it is desirable. When you use 1/4-20, you are making removable wing with no chance of minimising damage in a crash . If all you want is the capability to remove the wing, this works well. If you want some crash protection, go to much smaller bolts.