For a given span, simple flat wing-tips can actually have less drag then rounded ones as gently rounded shape (as seen from the front) actually encourages the wingtip vortex to wrap around the top of the wing. Something like a Hoerner tip (as if someone sliced the wing with a knife at a 45-degree angle) does provide slight benefit but I'd be careful about adding much to the wing span. A few inches here or there is not important, but if you add full semicircular tips (like a Cub) to an existing wing you've just upped the span from 23' 6" to 28" and that's pretty significant in terms of spar loads, strut placement, etc.
