I do ninja kills only if they are in range. Since I sleep and freeze until they are crippled, I use Phantom Dart and Swiftcast Flare when an enemy is low. I don't consider it stealing kills more than so overtaking or defending your position. I don't make it a priority to do so, but if I'm not able to Fire 3 > Fire 2 spam, I'll keep an eye open for low HP individuals to erase.