You don't need surprise when you have total air superiority. The B52 has now become a standoff weapon platform. Or it's employed when the risk of shoot down is minimal and the crew has access to friendly forces for search and rescue. We may see it employed for close air suport on stubborn Feteyen positions. I don't know if it has the precision or smart weapons for raids on Bagdad.
However, to answer you question. The military employs what's known as Altitude Reservations called ALTRV's. They keep a chunk of airspace avilable for their exclusive use. Generally, these amount to seldom used airways or altitudes.
If the enemy did have any air defenses, you'd reach a certain point, turn off your squawk or sqawk a certain code for interogators and go low level or declare Due Regard.