Steve..
          Strange that you shold ask, but only yesterday, that question was           asked and the supplier said that unless you really push them, they will 
  usually charge you for "even" layers because that is how they will process 
  the build. They will just etch away the even layer. They normally do not 
  buy single sided matl, so from a design/routing perspective, use the 
  layer and probably improve the design. Have a good day.

                                                  Bob Vanech