Depends on the mission profile. The nuclear penetration profile has changed in part from high altitude to one of low altitude penetrations. A friend of mine was a simulator troop at K.I. Sawyer and went for a ride-along, he said they never broke 500 feet during the entire mission profile, except for aerial refueling.
The BUFFs flying the iron bomb missions in Afghanistan are flying at high altitude (out of missile range) but they're still low enough to recognize, just by size and contrail.
The contrails of the B-52's are very visible over the cool skies of Afghansistan. Perhaps they would not be quite so visible in a warmer climate. Also, they are dropping a lot of JDAMS, which may require a lower altitude. (Not sure)
I was driving with a woman from Seattle to Montana a few years ago and stopped at a Columbia River Gorge vista point to admire the view. There were around 10 other people there and we're all looking down in this picturesque canyon when we hear a rumble coming from up toward the north. Just as I look northward a B-52 comes flying around a bend in the canyon and passes us....not above...not eye-level but BELOW - so we're all looking at the top of this monster as it flies by.
"Whoa" was just about the only thing I could say about it.