Perhaps a vertical winglet would give some kind of benefits! Sincerely, I doubt there can be important or noticeable gains with that, at least in helis they just use rounded tips. Of course, their blades work in a quite different way. A 45º bevel cut may be all that's needed, but after all it's easy to experiment with bolt-on wingtips or tip shapes...
As for me, I'd rather make a "whistleng tip" so that when I make descents with the engine at idle I can enhance that blade sound which sends shivers down my spine and makes me smile (something between [>:] and ).