For IAD, you've next to no real chance of seeing anything recognisable - the airport is too far from DC (much like STN in London is too far from most of London to get the kind of views you would get on approach to LHR.

On arrival at IAD, positioning is all important - keep as close to the doors of the buses that take you over to immigration as you can, so you have the best chance of making the front of the queue!