Your vocals are likely to come out sounding like they've been "pasted on top of" the mix.
Mixing is the art of getting all of the elements to sit together well. Pretty hard to properly mix the music tracks absent the vocals and expect there to be no conflicts when the vocals are added in. There's simply no good reason to do things this way.
Just give your mixing engineer a very clear idea of the vocal sound you are looking for, then listen to his/her preliminary mix and ask for any needed adjustment in the sound.