The term most people use is appears to be 'oil canning' - after the effect that happens to an oil can when empty.
It's a normal thing that happens to most aircraft structures that use formers, stringers and skin (semi-monocoque) construction.
The B-52 shows it quite clearly just behind the cockpit, and you can see it happen a lot if you watch closely.
If you see large and permanent evidence of it, then I'd be suspicious of the airframe, otherwise, not normally a problem.