Spherical cows on the other hand have to move with orbital velocity at least, or they fail to stay in vacuum for long and splash.
It takes a stupid cow, but when they can climb mountains as they do it is not inconcievable that one can roll down.
(I was suprised to see a cow jumping up on a ~3m rock ledge like it was nothing)
However, they can go up stairs, but not down them.
Considering that where I saw it they were doing that unsupervised since at least ~200BC (this being the Illyricum and then Dalmatia), I don't think they have any problems climbing down now.
Cows don't roll - they are frictionless so just glide over the surface smoothly.