I normally leave my grasses up until they don’t look good any longer, which can be in December some wet, nasty years or February or all the way to early spring, like late March or earliest April when I do the first cleanup tasks around the yard in general. I am in the Northeast, in Zone 5. Many years I have enjoyed looking at them all winter long. It’s strictly your aesthetic choice, but I think they are beautiful well through fall and into at least some of winter, so experiment. Definitely cut back before new growth begins, or you will be snipping off the tops of new shoots.