Wow I was just made aware Luka wears blue nail polish. I always thought it was pink.
I personally believe it's very important, especially for Vocaloid, since they rarely show up without painted nails. Also, since it usually isn't much of a hassle to just go into your local drugstore/pharmacy and buy some cheapy nail polish to use, it shouldn't be too hard of the part of the cosplay to complete either.
On the other hand, I also think it'd depend. If you were going to a convention, I'd say it wouldn't really matter, but if you were planning a photoshoot, nail polish would be best.
Also, I like your username. ;D
Every time I look back on my signature,
I somehow get embarrassed.