Jack,
We do both for different reasons. It is controlled by the scale of the design, sometime the layer-2-layer plus CNC tolerance are too much for the design causing failures and sometimes we can't relax the need for minuscule copper lines. In other cases the design will have many large temperature swings causing solder mask stress with fine line spacing causing failures over time. I also need to add that copper transition area changes are not completely automated and require additional checking time so why add them if not required.
  
IMHO
Rod

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