i personally have always gone by the principal of smooth is better.
i set my diff to be as loose as possible without slipping, and tune my setup in other areas of the car.
a 12ths "feeling" on the track is very dependent on how smooth the diff is. just try putting some old balls & plates in and see how different your car is to normal.
some-one along the line probably does adjust their diff for setups.... but for me its just another thing to get wrong! :
Keep It Simple Stupid! :-* roflol