absolutely relatively fabulous

I am busily getting my Aviary Corridor (see post below) for performance, and the conductor wants me to lower the dynamics of the flute in the higher parts so it will balance with the voice. That got me wondering - are dynamics just absolute values and the performer will play loud, or will the performer adjust his dynamic relative to the rest of the group? I realize that I am playing a bit of the devil's advocate here - a performer would certainly know what to do. But what about if the piece (as per usual) doesn't get enough rehearsal for the group to become familiar with everything in the piece? What do you do as a composer? Do you just not worry about this, or do you try to balance your dynamics by using different markings throughout?