The most knowledgeable person I know when dealing with OG's recommended cutting them back at "the end of February/beginning of March" - that was his stock response for grooming/pruning any ornamental grass. However, he modified that to say that they could be cut back at anytime during the dormant period if the appearance got too ratty. There is minimal contribution to increased winter hardiness by maintaining the dead top growth. And in zone 7, that's not really a concern anyway.

So you can cut back at any time now but do so before the new growth becomes too apparent or grows too tall. And cut back to somewhere in the 4-6" range. Many gardeners have found that it is easiest to cut these large grasses back if you tie them up tightly then slice through the bundles with an electric hedge trimmer, small chainsaw or other tool of your choice.