I think it depends on the breed of sheep. For example the breed we keep mostly is Hebridean, a Primitive type, which are fine with eating quite long scruffy grass. They will choose for themselves whether they take the long stuff, or rummage around nearer the ground for green leaves. The point is though that they are pulling it for themselves, so they can take shorter sections of long grass and gradually eat the long stalks down that way. If the grass was very long and was cut for them, then they would only get long stalks which would tangle up inside. Sheep rip their grass as they eat so if it's precut they have nothing to pull against. Goats must be much better at chewing through stalks than sheep are.
For sheep which are being raised quickly as fat lambs for meat, the grass is most nutritious at a short length, which is why cattle are often used to bring down long grass to sheep height. Cattle crop their grass by wrapping their tongues around the stems and pulling, which leaves shorter, nutritious blades for sheep.
To me it seems a waste of grass to cut a paddock and leave it to lie and rot on the surface. Better to restrict access to a smaller area and make good hay for winter feeding from any pastures un-needed in the summer.