It depends. But first, I would drill through the LE all the way into the wing joiner, and anchor the dowels into the joiner with epoxy.

If there is a plywood brace behind the LE at the seam, AND your dowels go through it, you'll be fine.

BUT, if there's only Balsa, and/or you don't know what's behind the LE, I'd fiberglass it - otherwise, the weight could rip the dowels right through the Balsa LE - Remember, those dowels are the ONLY thing holding the front of the wing on, and the front is where 90% of the stress is.