I would wait until spring. I have a Ravenna grass that I have had for years. Last fall I cut it back and it is not doing well at all this year. I guess there could be other reasons why it's not doing so well this year but from now on I'm cutting back only in spring.

Don't forget how much winter interest grasses can offer!

Here is a link that might be useful: {{gwi:865225}}

Thanks for posting that pic Grass_guy...there has been some differences of opinion about whether it is ok to cut ornamental grasses in the fall. Most of those who feel it doesn't matter are in zones which receive very little, if any, snow.

In areas which receive quite a bit of snowfall/ice cover in the winter, it is fairly important to leave them standing.

The picture provided above shows the snow lying on top of the stems, while not covering the root base of the plants. Ornamental grass roots need to be kept on the dry side during the winter months and if they are sheared, the snow can settle down and keep the area moist for quite some time. This can easily lead to root rot and destroy some, if not all, of the clump.

I never cut mine back until spring and they all thrive.

Hope that provides a bit of insight.