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
Cosplays In Progress: Homestuck-Male!B&WKanaya Maryam, Arcana Famiglia-Felicita
Cosplay Wishlist: DRRR-Celty Sturluson, Sugar Sugar Rune-Chocolat Meilleure, Homestuck-Vriska Serket Post Accident, Tower of God-Yuri Zahard
"Failures do what is tension relieving, while winners do what is goal achieving." -Dennis Waitley