I just wanted to thank everyone for taking time from their busy day to answer my query.  I appreciate all the input. 

  It looks like the "final answer" is to spec impedance & tolerance, trace width and tolerance and spec a minimum layer thickness.  This will allow different fabricators to use their favorite lams & prepregs, and our drawings will be in spec no matter who makes the boards.

  Again, thanks for your help!  It is great to have some data to backup your proposal when you go to your manager to make a change.

    Regards,

        Brandon Luther

        Dataram