The deciduous types, like Miscanthus can be `cut back hard in February/March which is best, although there's nothing to stop you doing it now if you don't like the look of them at this time of year.

However, they can look wonderful with the autumn sun shining on them - or frost in the winter, so I usually leave mine until February, as BS advises! Just don't leave them too long - or it's all too easy to damage the new shoots coming through.

I would add to spritz's comments that the one you're growing which seems to be Miscanthus Giganteus is the least showiest through the winter, and does tend to collapse, and is probably best cut back as soon as the stems have died

and to add to that as a rule grass gets copped bye animals,trodden on or burnt in its wild state so its quite a tuff plant bye nature.