There lays a big problem with Vocal tracks, they are probably the hardest to mix of all.
You have to consider how you would EQ vocals, what effects to apply to it, how those effects enhance or take away from it, how much effect, etc. Does it drown in reverb? Does it sound too dry?
Then you have to ask yourself, "how am I going to place this in the mix?" Is it too up front? Not audibable enough?
I think the only answer to that is, "you have to develope an ear for good vocals".
So like the previous post states, there's no absolute answer.