: So is it really necessary to fill the holes before glassing?

No,,and one might say that not filling them will allow unthickened epoxy to fill the end grain and strengthen the wood in that very spot should a bb be shot right there. Or that 5% of the holes that do get an air bubble will enable the kayak to float that much higher when flooded,,,or,,uh,,oh golly it's plywood. If you don't want holes then find another way to join the panels so holes aren't drilled.