Does the contractor want to do the actual shade install as an add-on? Ask the contractor for a list of shade manufacturers he is familiar with. This will give you a hint about what he has done in the past. If the contractor works with an interior decorator, ask the designer the same question.

If he balks at wiring for a particular brand, make an analogy between this prewire and dimmers -- is he prewiring for low voltage or high voltage lamps?