I eat my scones the Devon way: the cream makes a perfect smooth spreading surface for the jam. If you do it the other way round then the jam all sticks to the cream and gets lifted off the scone surface and uneven.... it's also, it has to be said, more aesthetically pleasing to have a lovely red blob of jam in the middle of a cloud of cream rather than a big white blob obscuring the glistening jammy fruit.
Although having said that, it doesn't really matter so much just so long as it's clotted cream that's used!! I've had disappointing experiences in Sussex where a cream tea is served with a slightly lighter, more airy cream (is this a Sussex thing?) is used. It wasn't as sacreligious as squirty cream (OK for the tops of ice-cream sundaes and perhaps American-style pies) but any kind of light cream just doesn't have the oozing yet fairly rigid consistency that you want for a scone/cream/jam combo. The only air in a cream tea should be in the scones themselves... the topping should be a heavy indulgent delight .